There's a whole section called "ECU Tuning"
Browse through it, and if you still have questions, we'll be more than happy to help :)
A quick overview is: You buy a unit to flash the tune onto the ECU. The most common one around here is the SCT Xcal3. Then you buy a custom tune from a tuner. TurboTom on the forum and Travis from FSwerks are the two that I see most commonly used on this website. Then you datalog, and if you have a reputable tuner, they will adjust the tune until it's right for you. But that is a very, very brief ovierview of how tuning works.