If you want to get into growing plants, the MINIMUM you'll want to get is 80 watts of fluorescent light for a 55 gallon tank (which would be fine for growing most low-light plants like java moss, java fern, anubias, cryptocoryne), up to about 110 watts. Anything over this may cause algae in your tank if you don't do CO2 injection (which is kind of complicated when you're first learning).
You can probably buy a special kit to "retro-fit" your existing hood, or easy route is to get a glass top as dorkfish said, then you can buy single or double lamp fluorescent fixtures to lay on top.
If you're going to buy hardware store fluorescents, make sure they are at least 6700k (will say on the box). This represents the colour spectrum of the light--plants require a fluorescent of 6700 k (yellower light) to 10, 000 k (white light).