Vitamin C is an antioxidant that helps repair and regenerate tissues, and aids absorption of iron. Citrus fruit, potatoes, tomatoes, and red peppers are especially high in vitamin C.
Extensive research indicates that vitamin C plays no especial role warding off the common cold. (Nobel laureate Linus Pauling started the myth about vitamin C curing the common cold in 1970.) While dietary intake is important, vitamin C supplements are ill-advised because of their side effects, most notably indigestion.