Breaking a string is nothing to worry about... it happens all the time to all musicians and seems to happen regardless of the age of the string so the sooner you learn to change strings yourself the better. Someone asked the same question (which I answered) a month or two back so have a look for a thread on this forum called something like "How do I change a string" or similar. You can't repair a string by the way, when it's broken it's broken.
If one string goes without excessive provocation the others probably aren't too far behind so I'd suggest you change the lot while you're at it. You could keep all the strings on until they break I suppose but they'll sound terrible, won't stay in tune and learning to play guitar under those circumstances won't be much fun. If you're not sure which brand you want I'd suggest a well-known brand name like Ernie Ball (personally I like these), Fender, D'addario or similar. Costs will be around $5 for bargain basement strings (best avoided) and maybe twice that for a good quality brand name. As for gauge, medium light is a good starting point. I also suggest you buy a couple of sets and make it a habit to change your strings at least every month if you're just hacking about on your own, more often if you're playing with other people.
Good luck
Mick