"Avoid sexual immorality." 1 Thessalonians 4:3 - 4
Be grateful that God made you a sexual being. Sex was not a mistake He made when He ran out of ideas. When Adam first saw Eve he did not say "I will bet she has got a wonderful personality!" No, he said, "This is...flesh of my flesh" (Gen 2:23). But when we do not understand God's original intent for sex, bad stuff happens. God does not want to destroy our ability to experience our sexuality. No, He wants to teach us how to enjoy it.
Saying "I am reserving sex for marriage" tends to raise eyebrows and get you excluded from the guest list of a lot of parties. So be it! When there is physical intimacy without true commitment, count on it; somebody is going to get hurt. You need to resolve in your heart to read God's Word and follow His directives on the subject. And you need to do it now, before you get into situations where you are tempted to compromise your character, because then you have waited too late.
We all struggle at times with our sexuality, particularly in today's culture. Sex is such a deep part of us. And guilt about sex has a way of making us feel separated from God like nothing else. So in order to determine your values and set some ground rules, you need to vow before God, "I am not going to allow my impulses to dictate to me, or sin to separate me from You. I choose to keep Your standards, to rely on Your Spirit to give me strength day by day. And if I do sin to seek Your forgiveness, get back up and move closer to You." Count on it - that is a prayer God will answer!