The the question is 'how are we to serve Him? What does it mean to serve Him? He did give us the conclusion to these questions in Ecc 12:13-14.
Let us hear the conclusion of the entire matter: Fear Elohim and guard His commands, for this applies to all mankind! For Elohim shall bring every work into right-ruling, including all that is hidden, whether good or whether evil.
Now considering these words were spoke long before Messiah appeared on the scene in the flesh, there should be no question as to what commands He was speaking of. And if that is not enough, one only needs to go to Acts 7:37-39 to find out what they are.
I believe that to be more specific as to what Christians should do.