When I was 14, I started having periods. At that time, I was observing the first Ramadan in my life, and I did not break my fast during my period. Until now, I did not make up for the days of fasting that occurred during my period. What should I do?

All perfect praise be to Allaah, the Lord of the worlds. I testify that there is none worthy of worship except Allaah and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and messenger.

A menstruating woman's fast is not valid. The Prophet, sallallaahu ‘alayhi wa sallam, said (asking women): “Is it not true that if she (a woman) has menses, she is not allowed to fast or pray?” They said, “Yes!” [Al-Bukhari and Muslim]

Hence, you are required to make up for the days in which you were menstruating. If you cannot remember how many these days were, then observe a number of days that you believe you have most probably missed and through which you will be most likely cleared of the obligation. You are also required to pay an expiation for delaying the observance of these days if you were able to observe them during previous years. The expiation is an amount of food that weighs about 750 gm. The food should be of a type that is commonly consumed in your home town. Recipients of the expiation must be poor Muslims. You pay the expiation for every day of fasting that you are required to observe and for each year that passed without making up for the missed days. You could observe the expiation before, after, or while fasting. 

Allaah Knows best.