Thou shalt know ... - David dissembled (compare also 1 Samuel 29:8), hoping, no doubt, that something would happen to prevent his fighting against his king and country.
Keeper of mine head - Captain of his bodyguard.
Related Commentaries of 1 Samuel 28:2
1 Samuel 28:2
2 And David said to Achish, Surely thou shalt know what thy servant can do. And Achish said to David, Therefore will I make thee keeper of mine head for ever.