Complex sentence: She began her career as a singer in nightclubs and performed in musicals in off-Broadway stage, while simultaneously studying, before making her television debut in the CBS daytime soap opera, "As the World Turns" (1975--76).
Split into simpler sentences: She began her career as a singer in nightclubs and performed in musicals while simultaneously studying. She did off-Broadway stage, before making her television debut in the CBS daytime soap opera, "As the World Turns" (1975--76).