That's a great question. In the middlegame, it's time to develop a plan, and also be prepared to change it.
One of the best ways to figure this out is through example. I have a bunch of videos where I play players rated under 1000, so you can focus on the opening - middlegame transition in those to get a feel for it.
https://www.youtube.com/playlist?list=PLeLmy4hisTQG7qb5ELyUYTKz2-i2WRzrh
So I develop e4/e5, then knights and bishops, castle, connect rooks but then I'm stuck. If my opponent played a good opening, I can't attack any pieces, and I basically just push pawns until my opponent blunders. Is there any tactics to force my opponent out of a defensive position and make them hang?