New Planted Tank

Hi there!
I've got a 20g high, it's a very nice size.
Check the wattage of the light that comes with the tank's canopy, I doubt it's going to be enough for a planted tank. For plants you want at least 2W per gallon - 40W - which is more than a standard fluorescent 18" or even 24" bulb provides. If you can find a VHO bulb that's the right length you might be able to have enough wattage with the fixture that comes in the kit - ask at the store, there should be an employee who can recommend lighting for plants. You can make a DIY CO2 injector easily, do a search and you'll find lots on lighting and making your own CO2 injector.
DON'T buy plants or fish when you get your tank, you'll have lots to do when you get home just setting it up. When everything's set up, you'll need to cycle the tank (do a search on the aquaria central site and forums for 'fishless cycle' and you'll find LOADS of information on how to mature your tank and get it ready for fish - this can take a few weeks but it's worth it).
You're going to need a substrate for the tank as well. If you're going to have plants, you might want to do a bit of research, since many people use more than just regular gravel for their substrate in planted tanks. I'm converting to planted and planning to use flourite, it's not the only option but I chose it since it seems the best all around choice.
The other things that come in the kit are fine, I like bio-wheel filters myself but some people aren't fond of them. If you're going to do a planted tank one thing you don't want is an under-gravel filter (just in case someone at the store suggests it).
You will also want to get something for changing the water - even just a piece of hose and a bucket will do, but a gravel vacuum or, even better, a Python will make water changes and keeping the tank clean easy. The easier it is, the more often you'll do it, and the happier the fish will be
The only other thing I suggest is to research before you buy - make sure the tank is the appropriate size for the type of fish you want to keep, that you have all the equipment you need, the right type/wattage of lighting, substrate, etc. so that when you dive into the hobby you will end up with a beautiful result and no regrets. Ask lots of questions, everyone here is really helpful. Have fun!

30-40w would be best, but with a "high" tank, you'll need to err on the upper limit of wattage. Better just to find a normal dimension tank if you're looking for plant growth. Besides, that way you have more surface area to grow plants.

No, fluo. fixtures are run on ballasts which put out a fixed amount of power and will therefore only run bulbs requiring that amount of power. If you want more wattage, you gotta add more bulbs. It's an investment, but for about $100 you can have a ton of light over your tank if you're handy. A little more if you've got to buy pre-built canopy.

DIY is better. In terms of quality, they're about equal (especially since they run off the same stuff). But for cost, need I say more.

If you've got money to spend, Fluorite is the Cadillac of planted substrate, but spend money on lighting first. The best substrate in the world won't replace adequate lighting. Get enough substrate to make a 2-3" layer. You can also use Profile Aquatic Potting Soil or Turface instead of Fluorite. They're very similar, they last ones are just a little lighter (both in colour and weight) than Fluorite, so it is a little harder to keep the plants in, but still completely functional and for all intents and purposes, just as good.

I agree with the wattage recomendations, but if thats not possible I've found that using a plant substrate like the Shultz Aquatic plant soil helps out a lot. What I do is put the plant soil down then put a layer of sand over the top to prevent the aquatic plant soil from clouding the tank. Due to the increase of nutrients in the substrate the plants seem to be able to cope better with lack of perfect lighting.

Oh, yeah, something I discovered about a year ago in my planted substrate search:
Schultz=Profile