Improving Your Chess - Resources for Beginners and Beyond.....
https://www.chess.com/blog/RussBell/improving-your-chess-resources-for-beginners-and-beyond
Good Positional Chess, Planning & Strategy Books for Beginners and Beyond...
https://www.chess.com/blog/RussBell/introduction-to-positional-chess-planning-strategy
This is a very important question that needs to be answered.
In midgame you need to make a plan. Now you may ask how? Well, the answer is simple -- "Imbalances". If you understand basic imbalances of a position, you can use those imbalances in your favour. This usage of imbalances for in your favour is called strategy. And how you will execute the strategy will be your plan.
Basic imbalances are:-
Space, minor pieces, pawn structure, material, initiative.
If you want to learn this deeply, read winning chess strategies by yasser seirawan.
Not to be rude but you are only rated 400. You are blundering a lot. Strategy will be of no use if you eventually lose material.
Here is a recent video of mine. Maybe hearing me explain my thought-process might help give you some inspiration for how to play in the middlegame. I might also make some videos specifically on the middlegame, so stay tuned for those! 
Middlegames are tough because every situation is different, but one idea if you are unsure what to do is also to decide what your weakest piece is and then try to improve it.
I usually start off good and then have no idea bout what happens in between and then I’m in the endgame…