FIRST, you both need to confess your sin to God and ask for His forgiveness. This simply means admitting that you did something that was outside God's desire for you and asking Him to not hold it against you. This is exactly why Jesus died on the cross -- to take our sins away from us and make us clean.
You might be feeling like God won't forgive you because you've done what you see (or what you think God sees) as such a bad thing. Having sex before marriage is no worse than any other sin we commit, and YES God can forgive you of it. Here are 3 articles that I would highly encourage you to read about sin and forgiveness.
- Can God forgive me for getting too involved sexually? (BillyGraham.org)
- A Second Chance at Virginity? (ChristianityToday.com)
- I have committed [insert sin]. Will God forgive me? (GotQuestions.org)
SECOND, you both need to repent, which means turn away from your sin and pursue new choices in your life. For you two, it means you have to stop having sex or any type of sexual activity (oral sex, masturbation, feeling each other up). In God's eyes, all of that is sinful sexual activity that is outside his will.
This is where it gets tough. Usually once a couple has had sex, it's really, really hard (but not impossible) to not be sexually involved again.
The first part of repentance is asking God to change your heart. The change has to begin in your heart before it ever involves behaviors. Otherwise, the change will be short-lived.
You have to decide if you really, really want to stop. I've known Christian couples who felt bad that they'd had sex, but didn't really want to stop because they didn't think it's wrong or they just liked the intimacy it brought to the relationship.
THIRD, you need to start making some radical changes in your relationship. This is where is gets really, really tough. This is also where you realize whether you really want to stop having sex or just kinda want to stop having sex.
What boundaries are you willing to put in place to help protect you two from compromising situations? Are you willing to not see each other after 9pm? Are you willing to never be alone together, not even in a car? Are you willing to cut out all forms of physical affection, even hugging or kissing, and limit yourselves only to holding hands. Are you even willing to break up if you felt God was calling you to that.
These are all pretty extreme measures, but I think these are the types of decisions you have to make in order to radically change your relationship. I've talked with a lot of couples that want to stop having sex but don't really want to change anything else about their relationship. It's just doesn't work that way. You can't simply change one behavior (especially one a significant as sex) without changing the whole relationship.
Is it easy? Heck no! Is it possible? Absolutely. But it takes turning to God and some trusted Christian friends, allowing Him to transform and renew your heart, and making some pretty radical changes in your life and relationship.