What exactly is the process for buying the right condom

I'm not having sex any time in the near future, but since I go to a catholic school, they taught me abstinence. I probably won't have sex for a few years for personal reasons. I have no idea how condoms are sized and I'd be scared to death to get the wrong size. Can some of you experienced guys tell me how condoms are sold and what I should know, so I'm not frantic come the time?