Unless you have played awfully in the opening and are completely tied up there is almost always something to do in the early/middlegame, it's just a case of identifying it, finding a plan. A good advice is to look at each of your pieces, decide which is worst/doing the least, then move this to improve its position. If not, can you make a luft for your king which may prove useful layer? Or move the king itself to a better square? Or claim some space with a pawn move?
What and where to move if nothing makes sense

Tactics is what you do when there's something to do.
Strategy is what you do when there's nothing to do.
If you seem to have no really constructive move, try to identify your worst-placed piece and find a way to improve its position.
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

Tactics is what you do when there's something to do.
Strategy is what you do when there's nothing to do.
If you seem to have no really constructive move, try to identify your worst-placed piece and find a way to improve its position.
+1
In similar theme: plans change in the dynamic game of chess. It is very possible that one (or more) or your pieces were "developed" to good squares, but now that the action has left: they are doing little there; therefore, move the pieces into the fray
I saw an interesting video today that said: "when you have nothing else to do, push pawns." Seems like that and "improve your worst piece" are good advice.

I think I've heard that somewhere before (the pawn thing) but I don't know if I agree with it. Seems like double edged advice. Obviously pawn breaks are a very important way of increasing the activity of your pieces, but advancing pawns frequently is also a good way to destroy your position.

hey,
I sometimes find myself in an early game or midgame where there's just no real move to make. Every field is guarded by the opponent and if I'd move to a safe field I'd give up my guard on another position.
Is it better to attack or just wait till my opponent starts "doing something"? I'm not sure what to do in those kind of situations
You already got good advice, so let me try an example.

Now a quick word about pawn breaks. Pawn breaks favor the side whose pieces are better placed, but you often can't wait until everything is ideally placed. Sometimes you have a small window, lets say it will take 3 moves for your opponent to catch up to your level of development. That's the time to rush to play a pawn break (if one is possible).
Other times you have to be patient. There's no easy rule of thumb that will let you know the correct timing
Anyway, hope that gave you some useful ideas.

... "when you have nothing else to do, push pawns."
Very bad advice.
Pawns cannot move backwards. So if you can't think of a constructive move and push a Pawn instead, what happens if it turns out to be a mistake? Almost any other random move would have been better, since a Knight or Bishop can withdraw on a later move if its position turns out to be untenable.

... "when you have nothing else to do, push pawns."
Very bad advice.
Pawns cannot move backwards. So if you can't think of a constructive move and push a Pawn instead, what happens if it turns out to be a mistake? Almost any other random move would have been better, since a Knight or Bishop can withdraw on a later move if its position turns out to be untenable.
Ok good, I'm glad others think so too. I wasn't sure whether to outright call is BS because pawn breaks are important.
I remember hearing some story about Korchnoi at a tournament, and him watching a game and reporting that so-and-so will soon lose. When asked why Korchnoi said he's started to move his pawns, and the sign of a strained strategist is they start pushing their pawns (the implication being he was ruining his position).

hey,
I sometimes find myself in an early game or midgame where there's just no real move to make. Every field is guarded by the opponent and if I'd move to a safe field I'd give up my guard on another position.
Is it better to attack or just wait till my opponent starts "doing something"? I'm not sure what to do in those kind of situations
You already got good advice, so let me try an example.

Hey, I just played a casual blitz game where I used the knight thing I showed and it reminded me of this topic.
So to reiterate: the forward squares of the knight are firmly controlled by pawns, plus it has juicy weak pawn targets on the queenside. Therefore it's good to reposition the knight.

I completely agree with what everybody said bout "in doubt, push a pawn". When you don't know what to do, you should avoid non commital moves.
What you probably should do as well is offer a draw : it's very likely that your opponent does not know what to do either and agrees.
I you can't safely push a pawn, I guess bring your king closer to the center: being a couple moves early in the middle game allows you to better prepare for the endgame and show your opponent some King activity.
hey,
I sometimes find myself in an early game or midgame where there's just no real move to make. Every field is guarded by the opponent and if I'd move to a safe field I'd give up my guard on another position.
Is it better to attack or just wait till my opponent starts "doing something"? I'm not sure what to do in those kind of situations