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.
Before answering this question, we want to draw the questioner’s attention to the fact that the Muslim must be careful when it comes to his religion. It is a gross error to be too heedless to know about the advent of Ramadan except after ten days of its advent.
As for your case, you should make up for the days of Ramadan that had passed before your knowing about its advent.
With regard to the period that you knew of, then you should have fasted these days, but since you did not, you should first repent to Allaah The Almighty for this major sin in addition to making up for these days.
This is the opinion of the majority of scholars including the Four Imaams. However, they differed whether you must offer the expiation or not. Some of them said that you should offer the expiation as long as you abandoned the fast intentionally. However, some of them made the expiation contingent on having sexual intercourse even if the fast was broken by another reason. A third group of scholars said that one must offer expiation only when the fast is broken by having sexual intercourse, which is not the case in question.
In any case, you have to repent to Allaah The Almighty and make up for these days, and it would be preferable if any of you offer the expiation for each day on which he had sexual intercourse.
If you did not make up for the missed fasting days until the next month of Ramadan came, then you would be required to offer, in addition to what is stated above, the expiation for the delay, which is 750 gm of the common food in your country for each missed fasting day.
Allaah knows best.