An altar can be anything really, an altar is sacred space that one practices his and her own spells or rituals. The altar that is something special to you and your beliefs. Find things that have special meaning to you and you can use them on your altar. Thats is best solution i find helpful. Also you can put candles, gemstones, crystals, athames, cauldrins, figures, tiles, anything that is associated with deities, gods, angels and such There can be figrues of anything rather its animals or deity that you admire or worship.
I would check out youtube, youtube has some great videos of wiccan, or pagan altars. That can kinda help you get started.