No, you don't have to turn your back on all your old friends if you become a Christian—and in fact God may want to use you to point them to Christ.
After all, if you truly give your life to Jesus, you are going to find that you become a different person. The reason for this may sound strange to you right now, but I want to assure you it's true: God Himself will come to live within you by His Spirit. The Bible says, "Therefore, if anyone is in Christ, he is a new creation; the old has gone, the new has come!" (2 Corinthians 5:17). This is God's promise to you.
And if Christ does change your life, who will see it more clearly than your old friends? And who will they listen to more openly than you? Yes, some of them may laugh at you—but down inside, most of them will respect you for what you've done, and they'll know they would be better people also if they'd give their lives to Jesus. Don't let them drag you down or tempt you into sin—but ask God to use you to tell them about Christ.
Don't delay giving your life to Christ. No matter what your past has been, God loves you, and He wants to forgive your past and change your future. He loves you so much that Christ was willing to suffer and die for you. He also loves you so much that He doesn't want you to drift through life without any purpose or direction. Christ wants to change your life—beginning today.