Anything you do to build up your natural defenses against disease helps. Never eat anything that isn't power packed, keep away from most white foods that are highly processed. Be sure your protein is of the best with as little fat as possible, never fried until you are well. Salmon and eggs from chickens as close to free range as possible is best. Eat plenty fresh vegetables, and use lots of herb to flavor. Almost every herb has some healing qualities. Be sure to have some fermented food like yogurt and sauerkraut that isn't processed.
Vitamin C is a great healer, but the body uses only a tiny bit if you take a C pill, and disposes of the rest. Within a half hour it has no more effect for healing, it would take more of the C vitamin. If you buy powered C and use just a bit in all the water you drink, it will keep the C working to heal and not make your kidneys work to handle a lot of excess C. Your body will use the water to help heal, too. Also, it helps to drink lots of green tea.
These things help the body heal, but for strep throat, it really must have a doctor's care, also.