The 98 DX I am purchasing has a little bit of rust under the gas cap. Could this easily be fixed in my own driveway/garage or is it wiser to take to a professional? I am looking to learn how to do body work myself, so if their is a way that I can do it myself without making it worse... any help would be appreciated.

1. If it's surface rust than grind/sand and use metal etching. Use filler (don't forget to sand it) and primer at end.

2. If it's where a rust has eaten or made a hole, then best bet is to replace or cut out the rust and weld a new section. But since your right next to the gas tank be careful.

I would stick to number one.

Is their a specific type of filler I should use, or stay away from? Or are all fillers pretty much the same?

pretty much the same,unless you want those filler with different type of color...lol