Altars
Altars are an imporant part of modern pagan practice and are a good way to connect with your deities (I'm being broad here. I'm including gods, ancestors, land spirits, et cetera in this definition). I sometimes see people downplay the importance of altars to avoid seeming materialistic or like you need an altar and, honestly...I think you do need an altar, but not in the way most people envision one.
Most people envision a pagan altar in a specific way. Usually a table, shelf or other surface with an altar cloth, statues, and various tools and vessels for rituals and, while that is likely the ideal for a lot of people, I think this is too narrow of a definition. When I say you need an altar, I mean you need a space designated as "sacred", even if it's just for a few minutes, that allows you to connect to your deities. This can be your place at the dining room table, a windowsill with a plate or cup from the kitchen, a shoebox full of little items that you hide under your bed, there's plenty of options. It's more about intention than what is physically there.
Setting Up An Altar
This is about setting up a more "traditional", permanent altar. Here's an example of what I mean by "traditional", permanent altar (this image is of the Norse altar I share with my partner). This is the sort of altar most people generally associate with a pagan practice.
The first thing to do would be to determine where it's located. This can be any surface, really. Nightstand, top of your dresser, bookshelf, widowsill, et cetera. Make sure it's clean before setting up anything and, if your practice includes it, cleansing may be something you want to do as well. Of course, you should also know what you want the altar to be for. Is it for your ancestors? Is it simply for your religion as a whole (ie Wiccan, Hellenic pagan, et cetera)? Is it for just one god?
What Do You Put on an Altar?
What you choose to put on an altar depends on your specific practice, which deities are being included, and personal taste. However, there are some items that are very commonly seen on altars, regardless of what type of paganism is practices.
Altar Cloths
Altar cloths serve both a decorative function and practical function. They look nice and can make the altar feel more "unified" (kind of like how an area rug can tie a room together) and it helps protect the surface your altar is sitting on. Altar cloths don't have to specifically be made as altar cloths. I've used a rabbit pelt (pictured on the example image), a regular table cloth, piece of fabric cut to size, and altar cloths made specifically for the type of altar it's on. It can be difficult to find altar cloths in my experience, though I might just be a little picky. They aren't strictly necessary -- as mentioned in the introduction paragraphs, the only thing really necessary is that you personally decided that a space is an altar and use it with that intention -- but, they are very nice to have. Saves a lot of trouble wiping incense power and picking at dripped wax.
Bowls, Plates & Cups
...And whatever other vessels you might find out there. These are primarily for practical reasons. Altars tend to get most of their use during offerings and offerings need a place to go. While you could just bring a plate or cup (or bowl, or whatever) from the kitchen and set it on the altar, some people like to have special bowls (and/or plates, et cetera) that they keep on their altars exclusively for offerings. Some practices may also have specific things used in these circumstances, like drinking horns in heathenry, or a jug for lustrious water on some Hellenic altars.
To reference the example image, we have a drinking horn for more "proper" offerings we do (we will pour mead into the horn, take a sip, then pour it into a cup we bring from the kitchen and leave on the altar). The wooden bowl is for food offerings and the little blue bowl is for liquids. We have been using it to offer water every morning, which we pour into a houseplant before bed.
Of course, they also serve to make the space look nice. It really makes you feel like it is a space for you and your deities.
Representations of Your Deity/Deities
This can be pretty broad as well and tends to be pretty individualized. It really depends on the deity and what you can reasonably acquire. Before getting a statue of Apollo (which I just kept on my nightstand, I still lived with my family), for example, I had a little picture of him and Hyakinthos that I printed out and kept in a little box with a tealight candle and a piece of citrine a friend gave me.
That being said, statues are generally what most people want. Unfortunately, this can be pricy and a lot of deities are just difficult to find. Statues are relatively easy to come by for Hellenic pagans, for example. Apollo was $30 on Amazon and I had a gift card. Norse deities can be easy enough to find, but are typically pricy, with the low end being $60 USD. Some places sell small figures of deiites that are inexpensive, but don't tend to be very good quality.
The most common thing people do when statues aren't an option is either find a nice print or postcard online and use that, or print out depictions of their deity that they find online. Creating your own artwork is also a great option, because it can also be a meaningful act of devotion.
Candles
Pagans and witches love candles. Candles are a really good addition to any altar because the scents and candles can be meaningful, lighting them is a very easy thing to do as a daily devotion and they come in may different shapes and sizes. They range from plain candles you can buy at any drug store to specialty candles that cost, frankly, a little too much money. Candles can be purely decorative as well and used just to set the mood. Candle holders, in particular, can really add to the appearance of your altar as well, though I think aesthetic should be secondary to practicality.
Chime candles are popular with witches and people who practice magic as part of their pagan practice. They burn relatively quicky and are inexpensive. They aren't a very good option if you're using candles as decor though. They're typically a one-use item.
Tealights are a good option because they're also inexpensive and come in sets. Even the specialty ones tend to be on the inexpensive end. At least compared to candles in containers and pillar candles. Tealights don't last a very long time though. However, there's plenty of nice looking tealight holders out there.
Pillar candles are nice because they can last a very long time depending on their size. If you incorporate lighting a candle into your daily practice, using a pillar candle would work well, since it wouldn't have to be replaces as often as tealights.
Votive candles are also decent daily ones, since they burn longer than tealights and take up less room than pillar candles.
Container candles, typically coming in jars or tins, are popular because you can just buy the candle and don't have to find anything to hold them in (tealights are sometimes fine without a holder, but they don't last as long). They're the most popular type of candle to find when looking for specialty candles (ie candles for your specific deity) and last a pretty decent amount of time. They do tend to be more expensive than other types of candles though.
What type of candles you choose to get depends on whom the candle is dedicated to (or if it's decorative), purpose and personal preference. I wouldn't consider candles to be strictly necessary, but they are incredibly nice to have since lighting a candle and saying a quick prayer can be a low effort way to connect with your deities.
Incense
Incense is one of the more historically attested items on this list, with many cultures (past and presen) using it. Incense is easy to find and inexpensive and is another very low energy way to connect with your deities. Stick incense is the easiest to find and burn, but resin, cone and loose incense is also available. What type of incense you choose to use, as well as what scent, comes down to preference/deity/et cetera. As most things in paganism tend to.
My partner and I try to light frankincense for the theoi every afternoon, if possible. Like candles, it's a relatively quick and easy way to give a small offering to your deities.
Votive Offerings
Votive offerings are anything that isn't consumed. Typically, this comes in the form of items that remind you of the deity you're giving it to. A lot of the random items you see on altars are usually votive offerings that are given to a deity. My rule for votive offerings, so they don't pile up, is to offer them and say "this is yours until the new year" and, once that time rolls around, I either remove them or re-offer them until the next year. Whatever works for you. Of course, offerings are the most broad thing in paganism so, I don't have any specific suggestions regarding them.
Other Items
Some people, me included, will sometimes buy items just as decoration. Your altar is whatever you want it to be, as long as it is practical and suits your needs, it doesn't really matter what you put on it. As long as you are happy with it and your deities are happy with it (as far as you can deduce that), then your altar is fine. It isn't helpful to compare your altars to others. Devotion and offerings given sincerely matters the most.
Advice
I cannot stress enough that altars do not need to be anything fancy. The whole purpose is that they're practical. It is better to be sincere in your devotion to your deities than it is to have a lot of expensive statues and altar pieces that you rarely use.
I generally think it's good practice to keep pantheons separate, unless you do divination and you deduce that it is fine. Your practice is your own, but different pantheons have different practices and rituals associated with them, so it tends to be more respectful to keep them separate. If you have space limitations, that's really up to you to figure out. I had two altars on one shelf before, I just had all of one pantheon on one side and the second one on the other and directed prayers only at one half of the shared altar space.
People often talk about not placing two gods who don't get along in mythology on the same altar and that only makes sense if you think mythology is literal. It's fine. If you find out through divination and experience that two deities shouldn't share altar space, then that's between you and them. Generally, though, it isn't an issue. I don't know why this idea is so prevalent.
Do not focus too hard on the aesthetic. I'm sorry, but it's really LARPy.