Google goes a long way bud.
First, let your engine breathe better. K&N drop in filter will do just fine. CAI systems dont really do sh*t for our cars. The JLT CAI is the only one that show a decent increase, but for over $200, Id rather spend it on something better to gain 5-7 hp.
Exhaust, nice cat back and new mid pipe: X pipe or H pipe, cats or no cats.(your choice on which brands you like) visit YouTube and type in different brands, that's what I did.
GEARS is the best and one of the first things you should do for your Mustang. I dont know if you are Auto or manual, so choose between 4.10 or 3.73. If you ask which one is better.... well, do a search because that thread has died SO MANY TIMES.
After you have aquired those things, see how you like it and get back to us.
Good luck bro!