Hi, and welcome to the forum.
Please read the Home site (on the bar above) about cavy care before you get your guinea pig. They are NOT inexpensive, easy-care animals, and they are also NOT suitable for young children. Even pre-teens need supervision and help in taking care of them. They're also social animals, and do better as part of a herd, even if the herd just has two pigs in it.
You'll need an adequate sized cage (see the Home site above), an unending supply of hay to keep the pig's innards working properly and its teeth worn down, good pellets, fresh vegetables, and a vet fund. Dog and cat vets usually don't know much, if anything, about the care of guinea pigs, and exotic vets can be expensive.
Let us know if we can help you.