Depending on the deity and you, it can he arranged however you want it to be arranged, or have the specific God/dess help you set it up to what "feels right". Usually a basic altar is a picture of the deity and a candle in front of it. Of course, this is not what everyone thinks, it's just what I picture it as. Instead of a candle, it can be stones, or a flower you found or bought, an amulet or a piece of jewelry, even a little trinket of sorts. It's all up to you. Usually over time, an altar grows. More things can be added as offerings, or just things added to the altar. Instead of a picture, you can replace it with a statue. There are endless possibilities.
With altars you dont have a set of things you need. Your altar should be filled with things that connect to you. But I you dont want some advice on what to have on an altar I can supply you with some. But it may not suit you, so don't take is to seriously. You probably want to start off with the elements. Air could be incense, fire could be a candle, water could be... Well water, and earth could be some dirt. Or you could use candle colors to signify the elements. And don't forget about spirit that could like a white candle or something. Some energy crystals to help keep positive energy, like quartz or rose quartz. An altar tile with a pentacle, or and altar cloth with one would be a good idea. Candles are always a good touch. And maybe some oils or herbs to go along with incense. But like I said those are some ideas. Put things on your altar that call out to you. I hope this helped!