When picking out guinea pigs you want to look for a few things:
*Clear eyes, no discharge from the corners of eyes or any cloudiness
*Nose should not have discharge
*Mouth should not have saliva coming out of it, teeth should fit together nicely and not be super long or uneven
*Listen to the guinea pig's breathing, it should not be raspy
*Use your hands to feel the guinea pig and make sure their are no out of place lumps on the underside
*Be sure to take a look at the feet and look for any swollen, red, or open sores
I highly encourage you get more than one guinea pig. Guinea pigs are herd animals and really need other guinea pigs for companionship.
With that being said, good luck!