Middlegame - What to do after had developed pieces in opening?

I started playing chess 3 months ago and I feel lost in middlegame, what should I do in this stage of the game?

You must make a plan my friend! Any weak spots of your opponent? Any pawn breaks? Look at all your pieces, decide which is the worst/least effective and move this to a better square. Imagine what you would like to do and analyse to see if you can make it happen. All of this whilst checking for your opponents plans of course. 

Can you recommend some instructive books that help-me improve this things?

Hm I'm not much a bookworm. You mus hope that @kindaspongey sees your post - he is a library. 

Get Logical Chess Move by Mode. It's a great book.

Here are some reading possibilities that I often mention:

https://www.chess.com/article/view/how-to-play-positional-chess

https://www.chess.com/article/view/test-your-positional-chess

https://www.chess.com/article/view/do-you-really-understand-positional-chess

https://www.chess.com/article/view/get-ready-to-test-your-positional-chess-again

https://www.chess.com/article/view/can-you-pass-this-positional-chess-test

https://www.chess.com/article/view/how-to-calculate-long-term-advantages-in-chess

https://www.chess.com/article/view/learning-basic-pawn-structures

https://www.chess.com/article/view/learning-pawn-structure-for-chess-players-under-2000

https://www.chess.com/article/view/your-pawn-structure-is-your-friend

https://www.chess.com/article/view/more-pawn-structures

Simple Attacking Plans by Fred Wilson (2012)
https://web.archive.org/web/20140708090402/http://www.chesscafe.com/text/review874.pdf
http://dev.jeremysilman.com/shop/pc/Simple-Attacking-Plans-77p3731.htm
Logical Chess: Move by Move by Irving Chernev (1957)
https://web.archive.org/web/20140708104437/http://www.chesscafe.com/text/logichess.pdf
The Most Instructive Games of Chess Ever Played by Irving Chernev (1965)
https://chessbookreviews.wordpress.com/tag/most-instructive-games-of-chess-ever-played/
Winning Chess by Irving Chernev and Fred Reinfeld (1948)
https://web.archive.org/web/20140708093415/http://www.chesscafe.com/text/review919.pdf
Back to Basics: Tactics by Dan Heisman (2007)
https://web.archive.org/web/20140708233537/http://www.chesscafe.com/text/review585.pdf
https://www.chess.com/article/view/book-review-back-to-basics-tactics
https://static1.squarespace.com/static/5856bd64ff7c50433c3803db/t/5895fc0ca5790af7895297e4/1486224396755/btbtactics2excerpt.pdf
Seirawan stuff:
http://seagaard.dk/review/eng/bo_beginner/ev_winning_chess.asp?KATID=BO&ID=BO-Beginner

https://web.archive.org/web/20140708090229/http://www.chesscafe.com/text/review492.pdf
http://www.nystar.com/tamarkin/review1.htm

https://web.archive.org/web/20140708092617/http://www.chesscafe.com/text/review560.pdf

50 Essential Chess Lessons by Steve Giddins
https://web.archive.org/web/20140708100833/http://www.chesscafe.com/text/review534.pdf
http://www.gambitbooks.com/pdfs/50_Essential_Chess_Lessons.pdf

Amateur's Mind by Jeremy Silman
https://web.archive.org/web/20140708094419/http://www.chesscafe.com/text/ammind.pdf
https://www.silmanjamespress.com/shop/chess/amateurs-mind-the-2nd-edition/

Simple Chess by Stean

https://web.archive.org/web/20140708104258/http://www.chesscafe.com/text/review400.pdf

http://store.doverpublications.com/0486424200.html

5 Most Important Principles of Attack in Chess…
Spend some time studying the following instructive game with the goal of understanding the reasoning (i.e., the principles) behind each and every move White makes…go over the game (slowly) as many times as it takes until you understand all of White’s decisions…
https://www.youtube.com/watch?v=vPlzkrBzlko

Thanks guys but is impossible play chess in this site, it's boring play against players that r using engine to make them moves, thanks for the books i'll read them; but i'm closing my account and i'll not play here anymore.

I've never seen a single person use an engine I think he just doesn't like chess. 

True but it's a tough site for a beginner.

another thing i do is win or lose

You'll hardly find engine cheaters at your rating, they are too good to be at your rating!

R.I.P. @TinglingBrain

lets have a moment of silence

I have the same problem as well. I get stuck on what to do after developing all my pieces. Then, I just hope that my opponent makes some kind of blunder that I can capitalize on.

hmm... how do you know that they are using engine ?

I never use one.

Many (even the most) my games I won by this rule. )

But it's very low level of Chess playing.