If you're interested in live plants, then check out the Plant Forum. At the top is a Sticky with lots of great links to information on keeping a planted aquarium. Once you've read up on the information feel free to ask questions on anything that still isn't making sense.
Substrate is anything covering the bottom of the aquarium (gravel, sand, specialy plant substrate, etc), so a sand substrate just means you woul replace your gravel with sand. Pool Filter Sand is a great option for this.