What should a 17-year-old do who has not completed fasting the month of Ramadan?

The Sharia has made puberty a sign of the completion of intellect. It has signs: nocturnal emission (ihtilam), growth of pubic hair (inbat), and reaching the age of fifteen lunar years for both male and female according to the Shafi'is and Hanbalis, and this is also a view in the Maliki school, based on the hadith of Ibn Umar (may Allah be pleased with them both). Abu Hanifa holds that puberty by age is eighteen years for males and seventeen years for females.

Accordingly, making up the missed days of Ramadan is obligatory upon you due to your reaching the age of fifteen. If your breaking of the fast was due to sexual intercourse during a Ramadan day, then you are required to make up the fast (qada') along with expiation (kaffara). If it was due to something else and you did so intentionally, then you are only required to repent and make up the fast. If it was due to a valid excuse, then you are only required to make up the fast.

If you delay making up these fasts until the next Ramadan arrives, then you are required to make up the fast along with an expiation for the delay, which is feeding one poor person for each day.