I have 3, and I love them :) They make great pets.
Keep them out of any drafts. Away from windows, fans, air conditioners - they are very suseptable to ear infections, chest infections.
TONS of hay. I pile my cage with hay, they love it, thrive on it and it keeps their tummies full. They are sorta like horses, where they have to graze continuously to keep their digestive systems healthy.
As you can see, my Pigs love their hay...lol
They get Watermelon, Apples, not a big fan of oranges. They love their lettuces and carrots.
I don't do the vitamin in the water. They get plenty in their hay, their alfalfa pellets and the fruits and veggies I give them on a daily basis.
Clean their cage once a week. Clip their nails on a regular basis as well, and always check their feet for any type of piggy diseases - they are prone to bumble foot, but moreso if you leave them in a filthy cage where they are standing on filth.
I put my pigs in a rubbermaid tub when I clean their cage...
This is Bindi. I named her that because of that swirl on her forehead. She's my favorite.
Maple on the left, and Mochiatto on the right - arguing about a piece of scrumptious hay...
All in all, they are great pets. There are many websites out there on Guinea Pigs and what they can eat and cannot eat. There is a lot of information out there, you just have to search. Just Google.