Guinea pigs cannot manufacture their own vitamin C. Although guinea pig pellet food contains vitamin C it is sometimes not enough. It is very important to supplement vitamin C, there are several alternatives you may choose. Some owners use a powder or liquid vitamin C and add it to the water bottle, some Guinea pigs do not like the taste so if you choose this option keep a good eye on your piggies to make sure there getting enough water, especially if they share the cage with other piggies. The other alternative is fresh vegetables, "Caution" some vegetables are not good sources of vitamin C and can cause illness in your piggies. I never feed my Piggies any lettuce at all, I only give my Piggies carrots as a snack. Some owners give broccoli, carrots, fruits and the list goes on (Which is absolutely fine).
Guinea pigs should also be fed hay daily, I give my Guinea pigs hay cubes, they are good for them plus it gives them something to chew on for the benefit of there teeth. It is important that they are given hay every day to aid their digestive system, in addition to pellets. Pellets should be your Guinea pigs main diet so choose wisely wich feed to give your piggies, remember Vitamin C is a must. I purchase a feed from Purina mills called Mazuri Guinea pig feed, my pigs love it. You can normally special order it from any farm supply store that sells Purina mills products.
Guinea pigs of coarse always need fresh Water daily, they say Guinea pigs spit back in there water bottles, so of coarse fresh Water will be appreciated by your piggies. Remember the better you take care of your piggie friends the longer they will be with you.