Final answer:
The nurse should advise the 20-year-old woman to seek care quickly if she experiences bad headaches, calf pain, or changes in vision.
Step-by-step explanation:
The nurse should advise the 20-year-old woman who will begin taking oral contraceptives for the first time to seek care quickly if she experiences bad headaches, calf pain, or changes in vision. These symptoms could be signs of serious side effects like blood clot formation, which is associated with the use of oral contraceptives. The nurse should emphasize the importance of reporting any concerning symptoms promptly to ensure appropriate medical attention.