Its really not that hard. A 20 gallon is a great size for plants. You need to get good lighting, at least 2 watts per gallon. Your substrate should be 2 1/2-3" deep and it would help a lot if you mixed in or put below the existing gravel somekind of aquatic plant soil. Fluorite is good but a MUCH cheaper and just as effective alternative is Shultz aquatic plant soil at home depot. Get a package of fertilizer tabs to stick into the gravel and a bottle of flourish plant food. Thats all you need for healthy plants. If you want more light in order to grow colorful plants you would have to do a DIY co2 setup. Its easy but not necesary. Great plants to start off with are java moss, java fern and anubias tied to driftwood, sword plants (I love ocelot swords), cobamba carolinia, hornwort, crypts, water sprite, ludwigea for some red, hygros, vals, sags, pygmy chain sword is great for the foreground.