Before you start spending money on your car, check your car's specs. DO NOT buy any parts that is not design for your car, and don't try to modify a part that is design for a different model.
Think about what you want from your car: speed, handling, look, balanced or max performance. If you want only speed out of your car, concentrate on the engine page. If handling is what you are looking for, start with the suspension page. If you don't care about speed but you want to look cool, try aero page and rims/tires page.
Try not to believe too much to advertisements.
Take advise from those who have similar experience.
Don't be too cheap, there is reason why the good ones cost more.
AND finally, GOOD LUCK, hope you enjoy modifying your car as much as I do. :-)