It is not possible for a believer to not ever sin after having believed in Jesus.
The key to this is in 1 John 1:6-10:
"If we say that we have fellowship with Him and yet walk in the darkness, we lie and do not practice the truth; but if we walk in the Light as He Himself is in the Light, we have fellowship with one another, and the blood of Jesus His Son cleanses us from all sin. If we say that we have no sin, we are deceiving ourselves and the truth is not in us. If we confess our sins, He is faithful and righteous to forgive us our sins and to cleanse us from all unrighteousness. If we say that we have not sinned, we make Him a liar and His word is not in us."
This means:
1. We have sinned, and it is still possible as a Christian to fall short.
2. Even though we can still fall short, we can confess our sins to God and He will forgive us, because of Jesus' blood.
3. We need to walk in the Light (diligently practice God's word in our lives) so that we are not purposefully walking in darkness (practicing evil). If we learn to practice what God says, then we are walking in the Light as Jesus Himself is in the Light.
This basically means learn to walk the talk. Even though we might still fall short, we can repent and keep on learning.