My 2 cents:
First, I don't know exactly what you meant by "tired quickly". After serving 1 basket of balls(50-70)? 2 basket? Also, when you are tired, are you feeling fatique in your whole body? or just certain parts of your body is tired?

If you feel fatique after serving 2 basket of balls(on a good weather day) then you probably need to do a little conditioning trainning.

Another possibility is that you are not breathing correctly or might be holding your breath too long. Always use the same routine when serving, even in practice, like always bounce the ball 3 times, 4 times, or whatever is comfortable to you before you toss it. Most importantly, try to inhale as you start to toss the ball(assuming your toss is not very high, just a little higher then your maximum reach), and exhale as you hit through the ball.

If you feel tired on your shoulder, or arm, then you are probably not using your body and especially your legs enough.