I can't sleep and I'm really tired?

I haven't been asleep yet and I can't sleep. I'm really tired. What's keeping me awake is that just before I drift asleep, my heart beats really fast and that scares me. Some other nights before, I've also noticed before I fell asleep there was this vibration sensation in my arms. I've just cried because I feel so horrible tonight. I'm only 13, can someone help?