NordStar is right, but I want to add on to what was already said.
Specific paths and religions call for specific things on your altar. So depending on what you are doing, there may be certain things you should have on your altar. Besides that, it is a very personal thing. I currently do not have an altar because my living conditions make it difficult for me to have one.
I just keep my tarot cards , pendulum, oracle deck, with taxidermy crocadile, bat an some crystals an candles on a table, but I dont think thats a alter. Becaues I am yet to be pick a path religion to follow! I was just curious, and thanks for the very great anwser !