Sometimes when i lean on my elbow or forearm my entire arm will start to shake. if i change positions i can usually get it to stop. what could this be from?

That can happen to anyone. The same leg shaking will occur if you are squatting long enough.

There are some Important nerves ( Radial and Ulnar nerves) that twist a bit at the Elbow and when they are compressed or pinched in certain positions, cause the sensations described. The Forearm can even go numb or 'fall asleep' . The best thing is to avoid those positions or change them as soon as one noticed the Tingling.