4. Therefore we are buried with him by baptism into death: that like as Christ was raised up from the dead by the glory of the Father, even so we also should walk in newness of life.
5. For if we have been planted together in the likeness of his death, we shall be also in the likeness of his resurrection:
6. Knowing this, that our old man is crucified with him, that the body of sin might be destroyed, that henceforth we should not serve sin.

Paul is saying that as you are baptized into Jesus Christ you are "buried" with Him symbolically into His death.  This means then that just as Jesus was raised from the dead, you also rise with Him and can walk in newness of life.  And since your old man or old nature is crucified with Him, you no longer have to serve sin.

The key is your identification with Christ.  You accept Him and what He did for you personally and you identify with Him in His death.  And as you identify with Him in His death and His resurrection your old sinful nature is dead and a new nature is alive!   Verse 7 says that he that is dead is freed from sin.  You are free from sin if you identify with Him in His death and resurrection.

Confession:

I am free from sin! I do not have to serve sin!  The old sinful nature is dead and I can walk in newness of life in Christ Jesus!