get those gears fixed!! Good bike maintenance is a MUST!! Otherwise you might end up in the middle of whoop whoop with busted gears and no way home!! Oh, and rocking (standing up on the pedals) is not a great method of cycling. It wears you out quicker and makes you less balanced and more prone to whoopsies.
Oh, and your butt will get better and more accustomed to riding over time. Invest in a suspension seat if you have the money- they're great. Or padded bike shorts
I love cycling. it's fab for the whole body