nd943,
Idealistically you're right, one should master the fastball and change before developing the curve, but many pitchers find the CHANGE -- not the curve -- to be the hardest one to master of all. I know that's true of my own son.
He's a LHP. For a very long time, his change stunk, and he had terrible command of it. He found that his curve really acted exactly the same as a change..after all, it's an off-speed pitch. And now that he's got a good change, he baffles and dominates. He's 14-4 so far this year, compared to 12-8 all of last year (with no change).
Importantly, though, DO NOT throw a curve ball without proper instruction from a qualified professional. I believe this needs to be hands-on with the instructor, not information read in print (or on the internet). An improperly thrown curve will hurt the arm and hang and be hit.
Finally, the above posters are absolutely correct.. LONG-TOSS, CORE WORK, NUTRITION, AEROBIC AND ANAEROBIC EXERCISE, COMBINED WITH PROPER MECHANICS WILL IMPROVE VELOCITY.