What Causes Shaky Muscle Of The Body?

Hi, I am 14 years old and I shake from every muscle of my body. This happens only when I use my muscles (not when they are in a rest position, like accosting on something or something like that). Exemple : If I put my hand horizontally in the air, my finger shakes.
I really don't know what happens and what's the problem, I don't like it.