I'm very thankful you have given your life to Christ. He begins to change us when we come to Him, and the fact that you feel uncomfortable over what you did to your employer is proof of this. The Bible says, "As for you, brothers, never tire of doing what is right" (2 Thessalonians 3:13).
What you did was wrong, and the only way to make it right is to go to your employer, tell him what you did, and repay what you stole (with interest). The Bible's standard is clear: Stealing is wrong, and "A thief must certainly make restitution" (Exodus 22:3).
Zacchaeus was a tax collector for the Roman government, and like most officials of his day was undoubtedly corrupt. But when he met Jesus he vowed, "If I have cheated anybody out of anything, I will pay back four times the amount" (Luke 19:8).
Make it clear, however, why you have now come forward: that you have given your life to Christ, and you aren't the same person you once were. Your boss may not be a Christian, but God will honor your testimony to him or her. Pray that your boss will be open to your action and will react in a forgiving way.
In addition, ask God to help you be the best worker you can possibly be. We witness not only by our words but by our works–and as a Christian, you witness also by how well you do your job. God bless you as you witness for Christ to those around you.