So, it's important to feed your guinea pig daily with a suitable dried food specifically for guinea pig as this will have the relevant amounts of vitamin c. Should your guinea pig not get enough vitamin c, the resulting deficiency may cause scurvy. Scurvy will cause dry and flaking sin with hair loss and should be treated by a vet.
Guinea pigs do have a tendency to flip their food bowls, so ensure you use a stable and heavy bowl, making sure fresh food is provided daily. When you change the food bowl take a few extra moments to ensure the water is changed daily also.
In addition to guinea pig mix, it's important to provide some fresh green foods, they will like a range of vegetables, broccoli and spinach are particularly good as they as a strong source of vitamin c. In addtion to the green vegetables you can also feed them root vegetables like carrots, and of course one of their favorite food, dandelion leaves, although these would be in short supply during winter months. A guinea pigs digestive system is not unlike that of a rabbit, they consume their own droppings in order to absorb the maximum nutrition. Again, just like rabbits it's important to ensure you make no sudden changes to a guinea pigs diet, they have a very sensitive digestive system and any sudden change can cause serious problems.
Guinea pigs are venerable to a number of commonly found plants, it's important that you keep then well away from such things as fox gloves, buttercups, ragwort, bracken, lily of the valley, and rhododendron. The flips side is they do like a special treat, and whilst there are a number of manufactured products available from pet stores, you can make up a bran mash, just mix a little bran with warm water and mash up.