Improving middle game

This is my best game so far. I think I'm improving openings but many times, I have no idea on what to do in the middle game.
For example, here I played 16.Qb3 but just to free the tower... any suggestion? Games to study?

Thanks friends!

Isn't d3 was a bit defensive? I thought we're supposed to play d4 if it's possible. And d3 blocks the bishop.

4 d3 is OK just like 4 d4 or 4 Bc4 or 4 g3 or 4 Be2.
5 Bd2 seems a bit passive.
12 c4 loses a pawn if he takes on c4 and then on e4.
14 Qb3 is not bad in itself, but 14 Qc1 attacking h6 so preventing o-o and seizing the c-file was more active.
18 d4 is wrong: you activate both his bishops

Good game.