What's a good move to make after the development of the knights?

So I usually start my game by moving in my central pawns, and developing with my knights, and normally my opponent does the same. What's a good move/tactic to go about afterwards? Do I open up the files for the bishops? Should I just attack the center pawn even though the knights are defending them? Or should I open the game in some other way?

Usually you castle and bring the bishops out somewhere, yes.

You've "completed development" when there are no pieces between your rooks on the back rank, so that means you've castled and all the bishops, knights, and the queen are off the back rank.

Yep.

Central pawns, knights out, bishops out, castle, move queen, get rooks on open files.  Do this while not hanging peices.   Now you are in the middle game, find tactics to get material advantage, and if not keep strong pawn islands and gain space.  Promote. Win.