Gears. Just find a combination that feels comfortable when you pedal. Switch to an easier combination just before going up a hill. As you ride more, you will develop a sense of which gear combinations you like for different circumstances.

Braking. Practice to get comfortable with front brake. It's a safety issue. The front brake is the one that will stop you when you really need to stop. Usually I apply both brakes, and often I'll apply more pressure to the front. I almost never use the rear brake alone.