To me, altars are a very fun thing to make. I like to rearrange them, add things, take things away. But some people are curious as to what to put on their altar, which is why this post was created.
Now, what is an altar? An altar is a surface or space used for religious, magickal, or ritual purposes. It is like your own working space, your circle, your haven. It may be directed to one deity, or just to magick. Maybe even a little bit of both? Everyone has a different view on the ways they set up their altars, and that's fine. There is not set-in-stone way. Now here's how to make or design one.
When it comes to surfaces, you can have a temporary altar that you pack up and set out whenever you need it (a suitcase/briefcase would be good for this. You can use a windowsill (be careful of flame hazards), you can use a cleared off table, a nightstand you don't use, even a cardboard box. It's all about you here. Use what you want and can use. No need for blowing your bank on some fancy table with shelves and all that.
You don't necessarily need an altar, but it is recommended to have one for many reasons. If you've read a few books, you'll see all these layouts and tools and all this stuff on the altar. Now I want to point out you do not need this. At all. These are simple templates you may or may not want to use. You don't even have to go by the book either. It's your altar, do what you want with it, place whatever you want on it. It's no big deal. However, there are a few things I'd suggest:
An Offering Bowl: If you're creating an altar for a deity, it would be handy to have an offering bowl on your altar.
An Incense Holder: These are good for just whenever you feel like burning incense, or for ritual usage. Since the smoke pleases benign/positive spirits and drives away the negative ones.
Crystals: I find placing crystals that are related to the deity or whatever you are making the altar for a great thing to have. You may use your favorite stone(s) or ones that go with the color scheme.
Candles: Candles are kind of a no-brainer on this one. You don't need them, but they add a little witchyness to it that I just love. They're also handy for ritual purposes or to be used in spells. You can go along with the color scheme for a little more aesthetic value.
Now the rest is up to you. You choose what goes on your altar, no one else but you. You can put all the tools you want on it, no tools, a lot of candles, etc. It is your altar, so it should directly relate to you, not something out of a book that you "have" to do.
I hope this cleared up a bit of the confusion, Be well.