Brian, my advice is to read a lot before you venture into plants, it can save a lot of problems later on. Try searching this forum, check out
www.plantgeek.net,
www.plantedtank.net,
www.aquaticplantcentral.com and lurk around for a while.
One thing that's very important is lighting, most tanks don't come with enough for plants (a 20g tank often comes with a hood/fixture that fits one 15W or one 20W fluorescent bulb). For a 20g, I'd recommend at least 40W of fluorescent light (2WPG), Java fern, Anubias, Java moss and other low-light tolerant plants may do okay with less.
I think It's a good idea to decide which plants you'd like to keep, then work the lighting, possible CO2 injection etc. around that. CO2 is important in tanks with higher light, but you can have a lower light tank with less demanding plants and not need to inject CO2, it's up to you.
Plants can be kept in gravel or sand, there are also specialized plant substrates like Flourite and Eco-complete. Some people choose to mix them with gravel, some use 100%. Plants like swords need a lot of iron, so using some Flourite/laterite is a good idea IMO, but not essential.
Don't be afraid to ask lots of questions, I've learned an unblievable amount from folks here and on other forums
