Hi! Welcome to the site!
I can understand your excitement in getting a guinea pig, but it's much better to do research first and plan EVERYTHING out before you bring them home. Since you already are on here and asking questions, you are off to a good start so kudos to you!
To start off, please get two guinea pigs. Guinea pigs are social animals, who live in large groups in the wild. They are sad and lonely by themselves. Despite how much time a human can spend time with a guinea pig, they can't compare to another guinea pig. We can't play with, groom, snuggle, sleep with, etc our guinea pigs like another guinea would. Could you imagine living in a world where you only lived with one type of animal of another species, by yourself? You can't communicate with them, and you sleep alone, eat alone, and spend your entire life by yourself. It be a horribly life, wouldn't it? That's why you should get two guineas, because they NEED friends. If you only have space for one, or only want one, then you need to either A) adopt a syrian hamster instead, as they like to be kept singly or B) find a rescue that has a guinea pig who needs to be kept in a single cavy home (they are rare and in between, but occasionally there will be a guinea pig that the guinea pig rescue says needs to be kept in a single piggy home)
That brings me to my next topic, please ADOPT and don't buy. Without going into paragraphs on why petco/petsmart/petland are awful places (you can read it about here
https://www.peta.org/features/under...ect-cruelty-filth-another-pet-trade-supplier/, video is graphic, but you don't have to watch the video, just read the text), I'll just summarize it. Petco and other pet stores sell animals for PROFIT. They don't care about the animals at all, and sell them to practically anyone who walks in. They encourage people buying animals on impulse, when they have no way to care for them. The animals are bred constantly, kept in horrible conditions, and rarely receive veterinary care. For that one guinea pig you want to buy, who knows how many died to get them there. Not too mention, most guinea pigs purchased from petco/petsmart are sick. Several people have had their petco purchased guinea pigs die from illnesses, or require very expensive vet visits to treat them.
Moving on from that, there are sooo many homeless animals (including guinea pigs) in need of a home in shelters and rescues. In the Rhode Island/Massachusetts area there are all these guinea pigs listed on petfinder
https://www.petfinder.com/pet-search?query_id=ED17B4F8-5CC4-11E4-95AD-5C626A2DE79B&page_number=0
As for a cage, cubes and coroplast cages are the way to go. They are incredibly cheap, and easy to make. Almost all store bought guinea pig cages are far too small for guinea pigs. A pair of females need at least 7.5 square feet, although 10.5 square feet is much better. A pair of males need at least 10.5 square feet, though 13 square feet is HIGHLY recommended. Read more about coroplast cages here.
https://www.guineapigcages.com/
Also, piggies can get sick, just like people. It's smart to have some money saved up BEFORE you adopt your piggies, in case an unexpected emergency requiring a vet visit (especially something requiring surgery) comes up. A typically vet visit seems to average around $40-$50 just for the appointment, and more for the treatment. I've never had a vet appointment cost under $150. Around $500 is a good amount to save up, but more is always better.
