What is the cure for double-mindedness?

Some people believe that "double-mindedness" is the worst of all human conditions. Even in the bible it says that "a man who s double-minded is unstable in all his ways". I think I kinda have this right now. Especially at school and in classrooms, I can't focus and my mind drifts ALL THE TIME. What is the true cure? Please give me real, good advice and nothing trivial.