If your new to modding, start off very simple. You can turn the car into something you hate very easily. I would avoid suspension mods unless you ride in a Mustang that is modded and experience it yourself to ground yourself on what it can do. In the least you should go with complete kits from a single vendor with a popular setup on which you can find tons of info. Beware of just getting a few bits and pieces here and there unless you are not concerned about performance and only care to drop the car for looks. All the pieces work together as a systems so just changing one or two pieces without picking the right ones will cause the rest to perform at less than optimum levels.
The easiest performance mods will be CAI, tune, exhaust and a shifter. Some simple style mods will be wheels, tint, stripes and body add ons like window lovers and side scoops if that is your taste. Check the vendors listed on the sight and just browse around to see what is available on the market. Once you see something that sparks your interest, do some searching around on here and you are sure to find numerous threads giving opinions and insight.
Enjoy your ride. First and foremost, make sure you know what you want out of your ride. Don't throw on a bunch of race spec parts if all you really want is a good looking cruiser. You're sure to spend way more money than you need to and won't enjoy it as much that way.
2013 Red Candy Metallic GT Premium Coupe
Black Leather, 6MT, Comfort Package, Brembo Package, 3.55 Axle
Borla Touring axle back, Suntec CXP 45% window tint, Eibach Pro-Kit, Steeda Strut Mounts, Rear Upper Control Arm and Panhard Bar, BMR Relocation Brackets, Energy Suspension Shifter Bushing