You always want to work on your velocity, but that shouldn't be your main concern at that age 14 and 15 because all grow into their bodies at different times. I was one of the fastest pitchers until that age and was probably the slowest then. About 16 on I was one of the fastest pitchers again. Work on lengthening your stride and gathering more momentum to home plate. You can Check out www.simplifiedpitching.com for more information.