You're going to want to first decide what you want to do with the car. Is it going to be a daily driver, a weekend cruiser, or am *** kicking track car? Your mods will be based off this decision. Certain mods can conflict with each other, and there's no sense in wasting money on things you don't need. For example, you're not going to want underdrive pulleys if you supercharge...superchargers usually run off the stock pulley.
Also do you want a stock looking GT with a ton of performance mods, or do you want to balance styling and performance out? Wheels really make or break the appearance of a mustang, but some aftermarket wheels can be significantly heavier than the stock ones...all things to consider.
If you're leaning towards a mildly modded daily driver or weekend car, exhaust, CAI, gears, and a tune are some good mods to start with. These will help you out in the power/quickness department. Lowering springs, CC plates, shocks/struts, brakes upper/lower control arms...all good supporting mods in addition to those power mods.
As I said before exhaust is always a good starter mod. It'll give you a slight boost in hp and much improved sound over stock...a CAI is another good mod. Usually on the newer models 05-10's a tune is required for the CAI, so as DDTCM suggested look into a custom place like Bamachips or VMP Tuning.
American Muscle is a great place to check out. Great products, great prices, free shipping, plus they are a site sponsor.
2010 Mustang Parts Categories: at AmericanMuscle.com - Free Shipping!
John at NQS will get you sorted out with suspension/braking, also a site sponsor: (you can PM him with any questions, he's good about responding)