Im worrying and my minds going crazy,please help,i need some good advice.?

I'm 14 years old, and all my worrying started about 18months ago. In the 18months I've worried about going outside, going to school, going on a holiday with my school, my friend was pregnant so i believed i was pregnant. All my worrying is driving me crazy, i can't concentrate in school, i can't get to sleep, i can't enjoy my activities that i do, im fed up of it all and just want my worries to go. My mind keeps wandering and imaginary things that are obviously not true, but i get worked up with it and start to believe what my mind is saying, how do i ignore my mind and is it true what the mind is saying? please help me,i just want my old life back the way i was.