Very simple - on both styles of bikes - always remember - little front, big back... easier to climb. Big front, little back, used to speed on flats. The key is to be in the middle and not cross-chain. MTB gearing tends to be more granny oriented. Just go out and practice until it seems right to you. I ride in the granny alot since I tend to spin up hills; going in a steady pace. If you like to power (which I recommend is not good for mountain biking) you may find yourself in the middle chainring more. I have friends who never move from the middle and seldom hit the large cassette in back. I also have many friends that just don't worry and have one gear.