is it brand-spankin' new or used?
If its used... 1st and foremost - research. 2nd - compression and leakdown test... you don't want to throw down a significant chunk of change on a STI and then have the engine go kaboom. scholars tell me that that is not a great feeling. 3rd - if it all checks out, get a feel for it and eventually begin to drive it like you stole it. From here, its all preference. You'll find people that don't mod, others that do.
If it's new ----> Boost.
Best advice I was given though by the people on here was to seriously read this forum as much as possible. Anything you want to know about the car can be found here. Oh and don't be afraid to reach out to people here and meet-up in person... most of us are all pretty chill and happy to help each other out.