You are probably gripping the string while drawing. This will cause the arrow to slide off the rest(bow shelf, hand). Use your fingers like a hookand pull straight back without gripping.
Start at 10 yards and shoot until you feel confident at that range. Then step back a few stepd and shoot more until you get confident then move back, etc,etc. Occasionally go back to 10 yards and try that again.
Also, pick a spot!!! Don't shoot at the target, find a small spot on the target and concentrate on hitting that spot.
In traditional archery consistancy is crutial like Youngbowyer said.