03 GT modifications

Just looking for advice on modifying my 03 GT stang. I guess im looking for the biggest boost for the least amount of cash. Im a novice when it comes to working on cars so im clueless on what is best. My first idea is an exhaust system (probably Flowmaster), but beyond that im stumped. Any info would be greatly apperciated.

Biggest acceleration uppage for the least amount of money=Gears

Biggest HP uppage for least amount of initial cash=Nitrous.