I got my two from the animal shelter. Somebody had seriously not been nice to them. They were six months old and overweight, with swollen red feet from their previous owners keeping them in a small wire floored cage. They were incredibly hand shy, and for some reason, somebody, probably a little kid, had painted their overgrown nails.
They've been with us for almost two years, and they live in a 2 grid x 4 grid C&C cage.
Guinea Pigs need unlimited Timothy hay, and quality pellets. They can choke on corn and seeds. They also need semi-annual to annunal check ups at an exotic vet, and should be spayed/neutered asap, as females can develop cancer if not spayed. My pigs get daily salads of carrots, lettuce, and sometimes other veggies. Guinea pigs need alot of Vitamin C, but some pig rescues discourage putting drops in their water, because it may discourage them from drinking.
Like someone else said, DO NOT put them in a harness, ball, or wheel. It can cause serious back damage, seeing as their legs are so small, their backs aren't very flexible.
Don't buy a guinea pig from a petstore. They come from disgusting conditions, and may have illnesses or mental issues. I don't know if I've forgotten anything. I hope I haven't discouraged you in any way from adopting a guinea pig. They're great pets, but also need proper care.