Originally Posted by LunaZ

Step One: Drive the car for a bit.
Step Two: Do some reading of the various sections of the forum.

You'll know where you want to start without us telling you.
Some seat time and research may tell you you want a louder car... or maybe a quieter car. Maybe you'll want tint and carbon fiber goodies. Maybe you'll want to lower the car or get bigger wheels and tires. Maybe you'll find the the car is too slow for you and you want to add forced induction or do the Mustang 5.0 mod.
Hell, you may even want to upgrade to a larger windshield washer fluid tank.
Everyone's personal taste is different and that determines how they mod their car.