How to stop really bad sweating??????? :(?

I'm a 14 year old girl and quite skinny, kind of healthy but get headaches like nearly everyday.. The problem is that I sweat soooo much and I don't know why!! The really bad thing is that you can see the sweat marks under my arms on my light blue school dress, so i have to always have my arms down or have a jumper on and its BOILING!!!!!!!!! I also always have sweaty clammy hands..... how do I stop sweating.. or what should I do??