I plan on buying a 2004 Toyota Celica GTS, what could I do to the car after I have bought it to make it faster? and what can I buy for it that would increase the hp?

SCC (sport compact car) fairly recently (in the last year or so) got a gts to run low 14's with just a cat back and a CAI.

id start there

Intake, Exhaust, Header, Forced Induction etc. etc.

it depends more on what your willing to spend and how far you want to take it