What goes on an altar? Well, that isn't an easy question; it depends on YOU. Some people like a full, traditional Wiccan altar setup with all the bells (no pun intended) and whistles, whereas others prefer just a couple of very special items. I fall somewhere in the middle for important work (such as a ritual, which I only do on rare occasions). Most of the time, my altar only has a few things on it: candles, my wood-burned pentacle my husband made for me for Mother's Day two years ago, my cauldron, and incense. Now that I have a small stone collection, I tend to use those along with my working candle wherever I happen to be when I decide to do something magickal, instead of going to my altar, though.
I apologize for rambling...
For YOUR altar, you need to think about what you want on it. Do you want physical representations for deities? If so, what? Do you want statuary, candles, stones, drawings...? Do you want representations of the elements? Ancestors? Other spirits? Do you want candles? Incense? A chalice?
There are probably hundreds of different ways to set up an altar, maybe thousands. Every Witch's altar will be different. When you decide to set one up, just think about what you will need to make yourself feel comfortable, peaceful, magickal, and etc. I won't go into every single type of thing you can put on your altar. If you've read ANY Wicca or Witchcraft 101 books, the author probably went into all the different magickal tools, sample altar setup, etc. If you haven't, it's not difficult to find information out about them. You can just google something like "Witch altar" and look at pictures to get ideas. If you've seen my YouTube channel, I think my first video showed my altar at one point, right around the time I had done a ritual so it was setup with the works, if you want to check that out for ideas.