SCT is a flash programmer or piggy back chip that can be custom tuned to support modifications to your PCM (powertrain control module read: computer) Mustangs can be tuned with a number of "chips" SCT and the Diablo Predator are the two most common.
You may get something out of tuning a stock set up, but look at it this way if you add a good CAI (cold air induction kit) and a tune you might get 20 RWHP, if you do the CAI and a pulley with a tune you might get 60-80 RWHP, my 03 Cobra went from 367 RWHP to 452 RWHP with CAI, 2.9 Reichard pulley and SCT tune. RWTQ went up to 479!!
Don't know where you are, but post a thread telling us where you are located and I am sure you will get a referral. Be sure to tell us which year and model car you have.