If you really do not know how to string a guitar you might want to take it to a local music store. They may charge you for restringing but it's worth it, especially if they let you watch. Just get some cheap Phosphor Bronze light (10's) or Medium (11's), you can try other brands every 3-6 months and restring yourself once you've seen it a few times.
BTW, if your local store is not friendly, helpful, and let's you watch - find another store or an experienced player who will help. Even if you have to pay an instructor, it's worth it in the long run. I wasted lots of money putting strings on wrong!