What goes on in your head? Are the thoughts you dwell upon God-glorifying, church-edifying, and helpful for your walk with Christ? Or are they not?
Our “meditations” matter. David said in Psalm 19:14 “Let the words of my mouth, and the meditation of my heart, be acceptable in thy sight, O Lord, my strength, and my redeemer.”
Not only do the words we speak have an effect, but also what we allow ourselves to think upon. Ask the Lord to help you think only on things that are true, honest, just, pure, lovely, of a good report, virtuous and praise worthy. 
Scripture taken from: Philippians 4:8