Ahh! Guinea pig talk, yay!
I had six all at once, so here are some tips. I haven't read through the thread though, so I apologize if I'm repeating some things.
1) Get two! They need friends, they're extremely social and do better in groups. Human love and affection is important, but guinea pigs need other piggies. I can type out the introduction process if you decide to get an unmatched pair, BUT for your first pair I really suggest adopting an already bonded pair. It will just make it that much easier.
2) They need a lot of space! 8 sq. feet is the absolute bare minimum for a pair, but this is honestly still very small. You will not find a pet-store cage large enough. The one that comes closes is the MidWest Canvas bottom cage. I really suggest building a C&C cage or making your own in another way. For example, a used a 16 square foot dog play-pen for my female group. There are lots of ways to get the space needed!
3) Never, ever use cedar bedding. Pine is okay as long as it's kiln dried. I really suggest using a mixture of fleece (with an absorbent layer) and kiln dried pine. I tried a lot of different combos before finally settling on that, and I think it works best.
4) Do NOT use a mixed feed. Get a good high quality basic pellet, that is timothy hay based (NOT alfalfa hay based). Oxbow is a good example. The mixed feeds are junk for guinea pigs. They're too high in fat and sugar, the colored bits are just food dye, and guinea pigs can actually joke on seeds.... which brings us to 4.5) Don't use ANY seeds. Treats with seeds are dangerous, as are those yogurt drops because piggies are lactose intolerant.
5) EACH pig needs a cup of piggy safe vegetables every day. Remember that you should have at least two, so you're going to be going through 14 cups of vegetables every week. The vegetables should be high in vitamin C.... bell peppers are a great source of C! Get the green ones, they're the lowest in sugar
6) They also need a long strand grass hay available at all times in unlimited amounts. They need hay to keep their digestive tracts moving constantly and to keep their teeth worn down. This can be timothy hay, meadow grass high, orchard grass hay, etc. It can NOT be alfalfa hay. Alfalfa is a legume, so it does nothing for their needs, and it's way too high in fat.
7) Guinea pigs need an exotic vet. Normal cat and dog vets are not very knowledgable in their needs, and some are not even allowed to legally see them because they're classified as exotic (which I learned the hard way when one of mine needed emergency vet care and was almost turned away completely).
Other tips:
-Use a kitchen area! Guinea pigs can't really be potty trained but a kitchen will help with cleaning. They poop most where they eat, so get a large litter tray to keep their hay, food dish, and water bottle. You can clean out the kitchen area more often
-Don't worry about them being over weight. As long as they have the proper cage space and are given floor time every day, and are fed in proper amounts, they will not become over weight. 1/8 cup of pellets per pig, 1 cup of vegetables per pig, and unlimited hay every day is the proper feeding amounts.