How can I improve my curveball?

I am a 17 year old male who is trying to play baseball my senior year. I intended to pitch. I throw a fastball accurately in the high 70's to mid 80's, and I have a good change-up. I do not however have a good breaking pitch in my repertoire. I have just never been able to get the right movement on my curve. I was wondering if anyone had any suggestions on the arm angle or grip. I throw at a high three-quarters arm angle.