Hi Steph!
Don't listen to your brother....listen to a fellow female
First, you gotta loose the idea of putting neons on that Mustang! This ain't no Honda, sweetie! You've gotta keep up the good name of Ford Mustang!
If you want it to sound good n' loud, you'll need a cat-back ehaust system...or at least an offroad H-pipe (you can keep your stock mufflers). You'll soon catch the speed bug that makes you wnat to go fassst...so gears would be next on the list. A lot of new owners get new shifters, too.
I bet you'll be happy (for a short while) once you do those few, inexpensive things. Talk to these guys on the board for types of exhaust and what kind of gears to get...I dunno too much about new GTs.
Take care! Good to have you here!