Gears: most people have a cadence, a rhythm they prefer to pedal at which feels comfortable. Try to pedal faster than that and it feels like you're jumping around and being inefficient, pedal slower and it feels like you're pushing hard. You just have to try it out and see what feels "right". It's a bit like finding a comfortable walking pace, some people take many short steps, some take long slower steps.
You use your gears to stay in a comfortable cadence for you. An uphill, or wind, or you're getting tired, you may want to shift to an easier gear so that each pedal stroke doesn't take that much effort. A downhill or a headwind, you may want to go to a harder gear so that you can put more effort into each pedal stroke.
Training: you could ride every day, but that wouldn't leave you much time for recovery, both from sore legs and a sore butt! But if you haven't biked much I'd say try to ride at least every other day, not too far, but increase gradually. A longer ride once a week is a good idea too, to get the feel of it.
Winter riding is much less about badassery and much more about bundle-uppery. - malkin
1995 Kona Cinder Cone commuterFrankenbike/Selle Italia SLR Lady Gel Flow
2008 white Nakamura Summit Custom mtb/Terry Falcon X
2000 Schwinn Fastback Comp road bike/Specialized Jett