I have to disagree. Many people jump into plants and lose interest. It's a waste of money to set up a large tank for plants if you're not positive that you will be up for the challenge of making it work. Plants aren't easy, and you have to be committed and you have to sacrifice a lot of time initially. Think of how much Flourite costs for a large tank. One bag for a 5 gal is plenty of money spent already. Not to mention the cost of upgrading the lights for a large tank. Also, small tanks are easier to inject DIY CO2 into. Large tanks usually need pressurized.
I don't think small tanks would be harder - a newbie would be more likely to stock it more full of plants, as opposed to a large tank, where they most likely wouldn't add enough plants to start. Also, water changes are quicker and easier on a small tank - more likely to be done often. And, the lower cost of fertilizing, and the option of using Flourish Excel aren't really viable in large tanks.
I would definitely suggest starting with the small tank, learning all about plants, CO2 and fertilization, then setting up a larger tank with all that knowledge to help you.
To answer the original question: A bag of Flourite, or Onyx sand (cheaper alternatives exist, do a search on the forum) would be really good. You'll need lots of light, 15 Watt fluorescent as djlen said. Nothing incandescent. Or a comparable amount in power compact fluorescent. You should also look into DIY CO2 (the yeast and sugar method) and fertilizers (Seachem's line would work well and not be expensive for such a small tank)
Good luck