The objective for both White and Black in the opening phase of a chess game is to quickly develop your pieces into a positional structure that maximizes attacking capabilities, and at the same time provides a solid defense. If at some point, you are not sure what to do next, see what piece still needs to be developed and where it best belongs.
Dr. Emmanuel Lasker’s rules for the opening (from Common Sense In Chess):
Do not move any pawns in the opening of a game but the King and Queen pawns.
Do not move any piece twice in the opening, but put it at once on the right square.
Bring out your knights before developing your bishops, especially the Queen’s Bishop.
Do not pin your opponent’s King Knight [against the Queen] before your opponent has castled.