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!
