Your post doesnt say wether you play acoustic or electric, if acoustic, then do as the above poster said, great advice.
If its an electric guitar you have, then the most used guage is 10s. However, they may or may not be the right ones for you. It all depends on a few things, if you are looking to play as fast as possible ove the fretboard, then smaller strings may be better, down to 8 guage, if your looking to do a Stevie Ray Vaughan impression, then your looking at 12s, or 13s and a lot more callouses!
As for what make of strings to buy, I've played for 20+ years and have tried all sorts, and to be honest, most of them sound fairly similar, go for a packet from one of the large manufacturers and you normally can't go wrong. I'd suggest trying different makes for a while, to see what you like, then stick with them.
As the previous poster said, if you can, ask someone experienced to restring the guitar for you.
Hope this helps.