We having the same spirit of faith, according as it is written, I believed, and therefore have I spoken; we also believe, and therefore speak; – 2 Corinthians 4:13 (KJV)
Most of us are speaking in order to believe. And meditation, muttering the Word to yourself, is important. It causes us to believe.
However the power comes when speak when we already believe.
I John 4:17 says as Christ is so are we in this world.
He had the power to speak and things happened. We have that power too, and we had before Jesus died.
Jesus said we could talk to a mountain, and if we believed, it would obey us (Matthew 17:20).
The question is what you believe positive or negative because whatever you speak, believing, will come to pass in your life. Good or bad.