Am I obligated to make up for and offer expiation for committing adultery before dawn on a Ramadan night and then breaking my fast that morning? And what should I do to repent of this sin?

You have committed two grave sins: adultery (zina) and intentionally breaking your fast in Ramadan. Both are major sins. It is obligatory for you to offer sincere repentance to Allah Almighty by avoiding places and people that lead you to disobedience. You must make up for the day you broke your fast. If breaking the fast involved intercourse after dawn, then expiation (kaffarah) is required: freeing a slave; if you cannot find one, then fasting for two consecutive months; if you are unable to do that, then feeding sixty poor people.