What Could Cause Excess Sweating Under The Armpits?

Hi, I have excessively sweating armpits. I go to school and I sweat so much and I've tried so much to stop the sweating but it just wont work. I am so embarrassed and ashamed that I have to put a dark jacket over my shirt to hide my sweat, but it goes through my dark jacket. People have referred me to many things but they just don't work. Please get back to me as soon as possible.