# The Lord said

Here "Lord" refers to God the Father.

# to my Lord

Here "Lord" refers to the Christ. Also, "my" refers to David. This means the Christ is superior to David.

# Sit at my right hand

"Sit in the place of honor beside me" 

# until I make your enemies your footstool

"until I conquer your enemies" or "until I make your enemies bow down before you"