Ok, first things first. Getting them on doesn't hurt. Really. Having your mouth that wide while they put them on might hurt your lips if they're chapped, but it'll just make your mouth dry. Nothing to it.
Braces feel really weird when you first get them on. And I'm not gonna lie - you really can't eat a whole lot the week after you get them on. Soft things, like pudding, jello, mashed potatoes, milkshakes, et cetera, is what you'll be eating for a while. If you're feeling brave, try thick noodles or something to that effect.
Once you get used to having braces, you can eat anything. Literally. Except the things the orthodontist told you not to eat. ((I'm going to let you in on a secret - your orthodontist will tell you not to chew gum. And you probably shouldn't. But. If you make sure the gum you want to chew is sugarless (ex: Stride, Orbit, Trident) it won't stick to your braces. Another thing you should ask about - I didn't, and I wish I had - is gum for people with dentures. Perhaps if they won't stick to dentures, they won't stick to your braces? Just a thought. ^_^ ))
Other than that, use basic common sense for food. You should be good. ^_^
You might get rubber bands to help fix your teeth after you've had your braces for a while. The orthodontist will show you how he or she wants you to wear them.
My teeth didn't move as fast as they should have because I didn't wear my bands. It cost me another 6 months. >.< Really, as much as a pain it is, be good and wear them as instructed.
In all honesty, braces really don't -hurt-. They'll ache or be sore when you go to get them readjusted, but I'm sure you'll be through it in no time. ^_^