Agree with the 20gal tank size. That is what I have currently and it's big enough to allow me to have enough fish to make me happy, but small enough to where maintenance isn't a pain to do. Although a lot of people say a larger tank is better for beginners, I personally feel that if you can master a 20gal or smaller sized tank, then upgrading to a larger tank won't be too big of a deal in the future
Most times you can find good deals on whole tank kits at places like Petsmart, Walmart, Petco, etc. However you can find equally good deals on a site like Craigslist. Just be careful with what you purchase on there. Some good advice I've gotten in regards to Craigslist buys include always going to the sellers home to inspect the tank for yourself before agreeing to purchase, and once you purchase used fish equipment from someone, take the time to sanitize using bleach or whatever you want to kill bad stuff that could be on that used equipment. Never just take that used equipment, set it up and then add fish.
Hope that helps!