An altar is a sacred space that you can set up, either indoors or outdoors, which serves as a sanctuary for saying prayers, casting spells, harnessing energy, meditating or to give offerings . It is usually a raised structure but, in the case of a temporary altar, it could be set up on the ground. I have used the top of a dresser, a coffee table or an end table, even a shelf on my bookcase. You can purchase something for this specific use but I prefer to use what I have on hand.
When I first constructed my altar I kept it very simple. I had a white candle, representing the Goddess, a yellow candle to represent the God, an incense burner and an offering bowl. I have since expended on this to include a bell (for calling the elements), an altar cloth, my wand (for spells and for casting a circle), a bowl of salt, a bowl of water and my chalice (cup). Below is a diagram of a simple altar setup:
I have candles for representing the Goddess and God (Spirit) but some people like to also add statues. The other items on the altar represent the four elements. It is your altar so you should chose the item that feel right to you. Here are a few examples of what you might use:
Earth: salt, clay dish, soil, rocks, seeds or a small plant.
Air: incense smoke, a feather or a fan
Fire: candles, lava stone, matches
Water: a bowl of water, sea shells or some sea glass.
An athame can also be added or used in place of a wand. An athame is a ceremonial knife used much like a wand for directing energy. It is not actually used for cutting.
Your altar is a sacred space, your temple. It is your place to honor the Divine and should be valued and cherished. It will change and grow as you do. Enjoy it!