Is just believing in God in your head enough to be saved?
No. A mere mental assent saying, 'well yeah, I believe in God,' is not what God is looking for. God expects confession with the mouth and verbalization, communicating to Himself and others. Scripture says, 'if thou shalt confess with thy mouth,' and a person must tell God out loud that they recognize from Scripture they are a sinner.
Les teaches from Romans 10:9-13 that salvation is not merely a head-level belief. Romans 10:9 says, 'if thou shalt confess with thy mouth,' because God expects you to communicate this not only to Himself but also to others. God is a communicator and expects us to verbalize these things. Les warns that too many people have only a mental assent, saying, 'well yeah, I believe in God,' and that is not what God is looking for. God is looking for a verbalization of all these various steps. In particular, you tell God that you recognize from Scripture that you are a sinner; tell Him out loud, 'Lord, I'm a sinner.' So a mere inward belief is not enough; the mouth must confess.
That if thou shalt confess with thy mouth the Lord Jesus, and shalt believe in thine heart that God hath raised him from the dead, thou shalt be saved.
More Questions
Is just saying 'I believe in God' enough, or does God want more?
A mere mental assent—saying 'I believe in God'—is not what God is looking for.
Do I have to say a prayer out loud to be saved, or can God just read my heart?
You should not merely assume God reads your heart (though He certainly can); He wants you to audibly say it out loud, calling on Him for salvation in words like 'Lord, I want you to save me. I believe that what you've done, you've done on my behalf.'