Guinea pigs should be fed similar to rabbits with a high quality guinea pig food and hay. Guinea pigs can have a larger percentage of pellets in their diet than rabbits, but they should have unlimited access to clean hay.
Like humans, guinea pigs require Vitamin C in their diet. Guinea pig food has some Vitamin C in it, but the amount varies with brand and how long it has been on the shelf or open. Therefore, it is recommended to supplement the diet with a fresh source of Vitamin C like oranges, peppers and tomatoes daily. Vitamin C supplements that are added to the drinking water are also available. However, the Vitamin C breaks down quickly in a water bottle. If using this supplement route, fresh water and Vitamin C drops should be added every day.
Guinea pigs can become very attached to one type of food. Therefore, it is helpful to expose young guinea pigs to several types of food before they develop a strong preference. This way you can switch foods more easily if a certain brand becomes unavailable.