Well, others will certainly want to add to this, but I would say that you definitely need:
- a filter
- extra filter catridges
- a top for the tank so fish don't jump out
- fish food, a net
- water conditioner (to remove chlorine and/or chloramine from the water when you do your regular water changes)
- and, (for tropical fish) a heater and a thermometer.
Then of course, you'll probably want substrate and something to decorate the tank with. I bought a very cheap bag of gravel for a few dollars but the decorations might get you since you're trying to do all of this for under $50. If you don't want to buy decorations for the tank you can look online and find lists of rocks that are safe for fish tanks. Here's a link
http://www.uniquaria.com/articles/sr.html to get you started.
Definitely check to see if Petsmart will honor their online prices in the store because it could definitely save you a few bucks if they do and you come prepared! One more thing, I'm sure you probably know this already, but just in case you don't: make sure that you let the tank cycle before stocking it fully and be sure to do regular water changes so that your nitrate levels don't get too high. You don't need to buy water testing kits right away because you can always bring a sample of your water into the petsmart and they'll test it for you for free.
Edited to add: hahaha, I'm such a slow poster. You all beat me to it while I was typing up my list!