Hi there streetrace94 and welcome to TorqueCars. You've asked a big question there - almost as large as "tell me about cars". it is probably best as prince says if you go through some of the articles on the main site.
The important thing is to know what you want - you can't have everything. Trying to squash everything into a few categories is harder but the following range from £10,000's to £100's (or lower).
MAXIMUM POWER - need to rebuild the engine every few months this is more of a track tune fully prepped race car.
Track day Car - some compromises are made and you have a noisy and hard riding car (not ideal for long drives with the ear and back ache it will give you!)
FAST road CAR - Probably where most of us are heading. A good well sorted car retaining reliability but with a good firm ride.
Lightly tuned - You tend to fix something your not happy with. For example you might improve the brakes or mid range power of the engine.
Then there is styling where the modifications are purely for cosmetic purposes.