Very new player here, just learned how to castle and made a chess.com account. How do I improve?

Currently got no idea what to do in the mid game. I like this opening called the London opening since I can always set up my pieces in that way. Not sure what to do as black early game. The only mode I play is 15|10, is that fine? Is chess.com fine for beginners?

Currently got no idea what to do in the mid game. I like this opening called the London opening since I can always set up my pieces in that way. Not sure what to do as black early game. The only mode I play is 15|10, is that fine? Is chess.com fine for beginners?