Proper posture and good technique are essential. And, as Sarah says, don't lock your left wrist. Left wrist should never be 'cocked up'...keep it as vertical and flexible as possible. Keep as light a grip on the bow as possible w/o dropping it. If you have any ear at all, you will know when your fingers in the correct position. Pick a tune and listen listen listen then try to play it.....find a good book with actual photographs of proper position and imitate.