What can I do to make up for the sex I had?
A view that you make up for a sin does not capture the severity of sin. Each and every sin that we commit brings on us a death sentence. "For the wages of sin is death" (Romans 6:23). If it was left right there, our situation would be hopeless because everyone sins (Romans 3:23). We can pay something back to make up for our sins because we already sold ourselves into sin and we have nothing left.
But God does want people lost to sin, so He took action to do what we cannot do ourselves. Jesus came and lived a sinless life and then died on the behalf of all mankind so that we might have an opportunity to leave sin. I can't pay back my sin, but I can submit to Jesus who then offers to release me from my sins.
I don't know your background, but the offer of forgiveness is made to people who decide to give up and and live as Christians. Thus, the primary thing is to do is become a Christian. Unfortunately, there is a lot of misinformation about what God requires of people to become a Christian, so even if you think you are a Christian, I invite you to look at:
If you find you haven't done all that God asked of you, then it is time for you to finish what you began.
If you truly are a Christian, then provision is made for Christians to fall into sin. You first have to repent of your sin, which means changing your mind about the acceptability of sin and change your behavior. "For godly sorrow produces repentance leading to salvation, not to be regretted; but the sorrow of the world produces death. For observe this very thing, that you sorrowed in a godly manner: What diligence it produced in you, what clearing of yourselves, what indignation, what fear, what vehement desire, what zeal, what vindication! In all things you proved yourselves to be clear in this matter" (II Corinthians 7:10-11). Then you need to confess to God that you've sinned. "If we confess our sins, He is faithful and just to forgive us our sins and to cleanse us from all unrighteousness" (I John 1:9).
None of this makes up for your sin. Instead it is the way we accept the offer of God to dismiss our sin.