elbow pain

I have been going out for baseball this year as a freshman in high school.
I have been a pitcher for many years in youth baseball.
I also quaterback for my high school team.
I have chosen to try out for 1st base, but when I extend my arm I have severe pain on the inside bone.
Is this tendenitis and what can I do for it?
If I don’t play baseball, how do I help it heal so that I can quarterback in the fall?