I was going to post the same Sheldon Brown link that DH did above. That is a great place to start, and there are lots of other helpful articles there. When I started, I didn't understand cadence (Sheldon's article explains it), and I usually rode in too high of a gear. As someone once explained to me, "If it is hard to pedal, make it easier. If it is easy to pedal, make it harder." You dont' want to spin your legs wildly, but you also don't want too much resistance.
In the beginning, I also did a lot of searching on you tube..."Shifting gears on comfort bike", etc.