Also known as a bubbler, the airstone sends tiny bubbles into the water. These bubbles agitate the water's surface which in turn keeps the water oxygenated. Without one, your water may get stale. Also, since harmful gas passes out of the aquarium through the surface, a steady current facilitates this as well. If you have a filter like the one described above, you could get by without one, but they're not expensive, so I suggest adding one to your tank as soon as possible.
Some fish use gravel for camoflage.
It also provides a place for good bacteria to grow. Although gravel comes in many colors, I suggest buying something without any dye added. Dye can pollute the water. Whatever you buy, make sure it is thoroughly rinsed before use. You want to remove any particles, dust, or dye.