The most important thing you can do is demonstrate by your life that Jesus has changed you. Your friends may doubt what you say – but they can’t deny the reality of a changed life. Jesus said, “Let your light shine before men, that they may see your good deeds and praise your Father in heaven” (Matthew 5:16).
After all, you were once like they are – and you’d probably have been skeptical also if one of your old friends had claimed they’d given their life to Jesus. But Christ is in the business of changing lives – as you have discovered. Don’t let their skepticism (or even their taunts) shake you, but thank God every day for His love for you, and ask Him to help you grow stronger in your faith. In time, your friends will realize that you have changed, and some may even become open to Christ.
The most important thing you can do, however, is to pray for your old friends. Only God can open their eyes and convince them of their need of Christ. Only He can break through their pride and self-centered ways of living and draw them to Himself.
As a footnote, I urge you to seek out a church where Christ is preached and you can gain new friends – friends who love Christ and can encourage and help you. You need them – and they need you.