Are you able to throw the ball over the net from behind the service line? If so, then you are strong enough to serve it over the net and just need to perfect your technique. Assuming you are right-handed, step forward with your left foot as you toss the ball with your left hand, then hit the ball with your right arm while stepping forward with your right foot. Serving the ball isn't all arm motion, you should also be moving your upper body forward. And tossing the ball consistently is important too. If you can't throw the ball over the net (or just barely), then you need to lift some weights to strengthen your arm.
the most power comes when u do a jump serve and hit it really hard. u need to get top spin on it or else it will just go out of bounds