I am a 17 year old boy. I come from a country with a very different religion and culture in which sex before marriage is common. However, I have met a girl from another country and different religion in which she has asked me to change my faith and belief, in which I am doing so. What if, although it is a very young age, we are thinking of marrying but not really married and end up having sex?
A country's cultural norms do not dictate what is morally right or wrong. As an example, Paul noted that in his day, Crete was noted by its own people to not be very good: "One of them, a prophet of their own, said, "Cretans are always liars, evil beasts, lazy gluttons" This testimony is true. Therefore rebuke them sharply, that they may be sound in the faith" (Titus 1:12-13). In Crete lying, evil, and laziness were common place, but it didn't make it right.
Sex outside of a marriage is wrong. See Why Sex Outside of Marriage is Wrong. What this girl is advocating is right regardless of religious claims or cultural norms.
If you do what is wrong and have sex anyway, you will be breaking God's law and will face His wrath. But you will also have causes a young woman to violate her beliefs, causing her shame. Likely she'll blame you for that shame and there is a good chance that she will reject you because you did not respect her.
Then how do I atone for my sins?
In a very real sense, you can't. There is nothing you could give God to make up for your sins. That is why God in His love for us sent Jesus to be our atonement. "My little children, these things I write to you, so that you may not sin. And if anyone sins, we have an Advocate with the Father, Jesus Christ the righteous. And He Himself is the propitiation for our sins, and not for ours only but also for the whole world" (I John 2:1-2).
God gives us a way to access the work Jesus did on our behalf. See What Saves Us From Sin?