You just gota ride. The more you ride the stronger you'll get.

The big gear in front and the small gear in the back causes you to go faster.
The small gear in the front and the big gear in the back causes you to go slower.

Go to a quiet road and just try out the gears.

Seat comfort: you will get tougher with time on the bike.

To get up the hill change into the lowest gear before you get to the hill.
After you gain some confidence you can stand on the pedals for more power.