Put stuff on it you need and you or your deities like. A statue of your deities is good, but you could use candles or a picture instead. If your deity is associated with an animal, you could put a bunch of animal figurines on the altar. An altar cloth is common and you change it with the season or holiday, but not mandatory, it's mostly decoration. You might consider keeping tools on or beside your altar so if you need it it's right there.
My altar has my god statue and a candle for my Goddess [have yet to find one for my Goddess] a bowl for water offerings and a dish for food offerings. In front of that I have an incense burner, and I add decorations, flowers and candles depending on the season. Simple but it works for me.
Also, make sure you keep your altar clean. It is a sacred space for your gods, to let it get dirty is disrespectful to them. So while placing a bunch of stuff might look nice, remember you will be cleaning each piece at least once a week, so don't go overboard.