Guinea pigs should always have a good daily supply of grass or hay. They also need to have guinea pig mix, mash or guinea pig pellets. If they don't eat something in the mix, don't leave it out of their next feed because it is not kind to allow them to develop lazy eating habits. They will generally eat the same things that we do but don't give them a lot of lettuce or spinach because it has little food value and is toxic to a guinea pig. Most plants that were grown from a bulb are usually toxic to a guinea pig, so don't feed them to them.
Potatoes are also harmful.
Guinea pigs are herbivores and so should not be fed meat. Guinea pigs generally know what is good for them and also how much is good for them but never give one plants you know are unsuitable and try to stick to plants you can be sure are safe.