Welcome to the hobby of planted-tank keeping.
Things you will need (considering that you already have the tank/stand/hood):
Lighting: Don't underestimate this one. Good lighting is necessary if you want to have a successful go at actually growing plants. I'd start by looking at AH Supply
Substrate: Look for something that is rich in iron, with a grain size that isn't too small. Very popular choices are Eco-Complete
Filtration: For a planted tank of that size, you will almost certainly want to use a canister filter. They are very efficient, easy to clean, and because they are "external to the tank" they allow you to plumb other devices inline in the plumbing. Popular brands are Eheim
Heating: If you choose a cannister filter, you may want to choose an "external" heater to keep equipment out of the tank. A good choice might be a Hydor
external inline heater.
Fertilizers: If you choose to go with a high-light type of setup, the plants will grow quickly and will need to be fertilized to provide the nutrients they need to outcompete algae. There are many options for ferts, but the most cost effective is dosing dry ferts. Greg Watson
is a good source for these.
CO2: A planted tank will require some source of available carbon for the plants. Whether you choose to set up a pressurized system, or go for a DIY approach depends entirely on your budget. Read some of the links below to learn more.
Other: In your planning, don't forget to consider other needed accessories (timers for the lights, a GFCI outlet if you don't already have one, Thermometer, Test Kits, etc)
To get you started, you'll want to do plenty of background reading about the subject of planted aquariums. Some suggested reading:
Rex Grigg's Guide
Chuck's Planted Aquaria Pages
Tom Barr's Estimative Index
Planted Tank Forums Search