First off a bow should not be gripped and the weight should not be held with your arms persay. Open up the bow hand to prevent torque and keep your bow hand slightly bend at the elbow with your wrist parrellel to the bow. If you can not do this then the draw length is to long and should be adjusted. It also may be possible that you haven't built up the strength in the muscles required to hold or pull a bow's weight. An excerise is to sit on the ground, with your legs straight out then draw your bow if you can't or it is terribly hard it is to much weight right now for you to accurately handle. (please be sure to nock an arrow and have a target positioned in a safe direction to stop the arrow in the event you have to release). Are you concentrating on the pin on the target, never try and hold the pin on the bull let it hover and release. If you find your shaking stop, relax and start over. I would also suggest you try shooting at 10 or 15 yards to get confidence and the feel of the mechanics involved in the release. If possible ask another archer to watch you shoot and point out what you may or may not be doing.

Keep practicing, remember the basics and if frustrated stop to regain composure.

Good Luck