Firstly, Congrats on winning!
3.Bc4 is not so good, better is d4, because that's why you played Nf3 in the first place.
3.Nd7 is terrible, you exploited it well but much better is Nf6!
5.g6 is random, Nb6 was better
6.0-0 is good but a better move would have been Bg5! Taking advantage to put pressure on the e-pawn
8.Nf3 was unneccesary, your knight's good there, don't move it until absolutely neccesary. Better is Nc3, good development
9.Bg5 is superb
14.Bb2 and you're far better off.
15.Bh6! is better, Rd1 is not neccesary, the other rook can go there later
16.b5 is a blunder, you were bound to win after that
18.Bf4 is better
19.Bh6? is a mistake, wastes a tempo
Nice tactic, but you could have gained a tempo with Bg7+! Kxg7 Rxd7+ Qxd7 Qxd7+
I just started playing chess very recently, and reached 1300 with decent speed. Suddenly my progress stopped, and I fell down to 1150 very quickly. I decided to grab some chess books and study for a week, and I feel like results are showing. Played 5 games since I came back, won 4 of them. The following is against the strongest opponent (1391). Any constructive criticism would be highly appreciated.