How do i throw with more velocity?

I play for my JV baseball team in my highschool. Last year on the Freshman team I did pretty good but I did not throw hard at all. When I pitch I'm good at putting it in location so I made the team. This year my arm was hurting and I was throwin like 40 mph on JV which is not good what so ever. Our first game they put me in and my arm did not hurt for the first time in a while. I had gotten my speed back up to about 55 but I was only in for 3 batters I got them out 1-2-3 so my coaches are happy with me now but if I want to make varsity and prove myself on JV I need to throw harder and faster. I'm left handed so I have a little bit of an advantage and my ball has what they call the lefty curve. I want to be able to throw harder and be very effective. I'm generally a reliever now in h.s. but I was a starter all my life and I want to get back to that