There are lot's of explanations on divebombing (check
http://www.netaces.org ), but what really makes it work is experience.
For a beginner, I would suggest going down straigt at a 90 degree angle - try flying inverted over your target until the target is in the center of your up view. Dive down straight onto target and release once the plane has settled into the dive, and don't use any control input some seconds before release - the bomb can't steer.
For more shallow angles, train with the magic crosshair on in offline mode (dunno where the toggle is ATM). It shows you where your bombs will hit, so in a comparable online situation, your judgement of the impact point will improve. Generally, at 60 degrees and low to medium speed, your bomb will fall to the point you see at the top of your engine cowling.