Usually when you have the more active pieces, you don't want to lock up the center. Rather you want it to explode open so that you can overwhelm your opponents position.
12...Qd7?? was a big mistake but you failed to capitalize with Nb6 winning the exchange.
as you noted, 15. b4???, Nxf6+ would have won you the game
19. Ng5 was unnecessary. You should know that a rook is worth 5 points while a knight is worth 3. Your opponent wouldn't (shouldn't) initiate that trade, if he did it would only benefit you. Better would have been protecting your weak b-pawn or maybe Rxd6.
after 19...Rf6 you could still capture the pawn since you would take his rook back after Nf7+
Nice mate to finish the game. You completely dominated your opponent who never had any advantage in this game.
Yeah although i have known chess rules since i was 5(and now i'm 15), i have know interested in chess for a half year.
This game has lots of plunders, so please, don't punish me with doing small mistakes..