How to improve

Does anyone know what's a good thing to improve in in general at around 600 rating? 

you could study and do some chess puzzles

Opoingin middle game end game

tactics, tactics, tactics. just don't do stuff in the opening like getting out your queen early. also develop pieces, make sure your pieces are protected. once you make sure that no piece is hanging, most of the time you will notice that you made the most of your tactics practice by naturally seeing your opponent's weak points. play longer games as well.

don't focus on learning openings too much. just stick to the opening principles and you'll be fine.

as for the endgame, it would be better if you would also work on this, much like your tactics practice. study king and pawn endgames, rook endgames, etc. this is so you would know how to convert an advantage to a winning game. glhf

Opening Principles is very important around this level. Why? Mostly because players under 1000 rarely follow them and also play "weird" lines or moves (usually because they simply don't know better), so memorizing openings won't work against them. 

https://www.chess.com/blog/KeSetoKaiba/opening-principles-again 

At sub-1000 chess.com rating, also important is puzzles/tactics because they can help you develop some pattern recognition - that is a fancy way of saying, solving them subconsciously gets your brain introduced to ideas that you might use in your own games. 

Finally, worth investing some time into is basic checkmates and basic theoretical endgames. Things like Q + K vs K (checkmate), R + K vs K (checkmate) or K + central pawn vs K (endgame) with the last one being won or drawn based on factors like King Opposition or how far advanced the pawn is.

At 600 you need to learn a bit of everything, rather than improve in a specialized area. The exception to that is tactics and using a tactics trainer.

For example, a basic beginner book will give you the endgames which you really need to know as a good basis and is better than focusing on endgames exclusively.

get good

A rating of 600 indicates frequent blunders. Always check your intended move is no blunder before you play it. That alone will get you to 1500.