Guinea pigs can bite at first... especially males. I started with three (all supposedly male, but thats another story) and quickly one establishedhimself as the dominant one; always first to food water etc, and quite skittish when handled, however as had been said, after a few days (or less depends on where you buy them: larger pet stores dont handle the animals as much so they are more likely to bite). Just be careful with sexing them; its extremely difficult to do for a few weeks, and thus lots of pet stores will get it wrong, and if they do, a female can have 3-4babies in a few months time. Also expect a fairly large cost: a run + hutch as startup isn't too bad, but they do eat constantly, and fresh food is vital for a healthy animal.
If your new to guinea pigs go for a short haired variety: longer haired animals will require daily brushing, as well as being less hardy in terms of disease immunity; though that being said all are quite hardy, the biggest problems I have seen in years of rearing them is a few colds (give them a tiny drop of whiskey and they'll be fine) + the odd death after pregnancy that can occur.
Good luck!