Carmichael's "food for fitness" is a really good guide.
Start with a good basic diet. Something like south beach is a decent start - fresh vegetables, fresh fruit, whole grains, quality protein. Limit the simple carbs and refined flour. Less processed is better.
This is all about managing your blood sugar. If you eat simple sugars (anything with a high glycemic index), your blood sugar goes up, your insulin goes up, you store the sugar as fat, your blood sugar goes down, and then you get hungry again.
During exercise, things are different. You are burning sugar, so extra simple sugar coming in doesn't have a bad effect. Something like 250 cal/hour is all you need.
When you start doing longer rides - say, 2 hours or more - a recovery drink/small meal is important as well.
Most people ride too hard when they start - try to be able to talk comfortably for most of your riding.