Make sure your arms are perpendicular to your torso, that is, have them straight out. Touch the bar to your chest if you plan to compete, otherwise, there is a rule of thumb regarding that. Keep your core tight and lower the bar in a controlled manner as far as you can without arching your back. Do it with a light weight first to understand good form. If you are tall, this may be higher than your chest, if you're short, well you can't into your chest so just keep it there. I'm of average height (5'10") and have a fairly large chest, so I touch the bar to it.
P.S. in order to protect your rotator cuffs, do some warm up exercises before performing bench presses.