I have one (Hamish) that I rescued from a relative whose kids ignored it and were allergic. Poor thing was left in the cold basement. The thing they were probably allegic to was the cedar bedding. It is full of added aromatic oils. It made me sneeze when I brought him home. Cedar bedding is not good for rodents. It can cause resperatory problems. I switched him over to Aspen bedding which is free of any oils and absorbs moisture well. I strongly recommend Aspen bedding to prevent future problems.
I totally fixed him up. He gained weight, his coat thickened, and he's doing well now. His cage is right beside my desk.
Guinea pigs need lots of gentle attention. They are social animals. Your best bet would be to have two females.
Their diet should consist of alfalfa pellets, Timothy hay, and a cup of fresh veggies every day. Don't buy the mixes that have seeds and other dried fruit in them. This causes selective eating (they don't get enough nutrition from the good stuff in their food) and they can also choke on some of the seeds in that kind of food.
Guinea Pigs love to eat. If they have Timothy Hay to graze on they will be quite content. Make sure it is placed in a pile in the corner and kind of flattened so your new buddy doesn't poke his eye on it.
Best to have a flat bottomed cage as wire bottoms are very hard on their little feet. Don't place the cage where there is a draft.
I've occasionally trimmed my boy's nails with cat scissors. I've also given him a bath on a couple of occasions.
Do not place sharp objects in the cage. My little guy poked his eye on some hay. I was able to clear it up with a special eye wash.
This site has lots of excellent information on diet and care.
http://www.aracnet.com/~seagull/Guineas/careguide.html