Surely what
@tpr suggests is going to improve your level. But just playing longer games is not enough. One chess game consists of more than variants that need calculation. What I suggest is:
1) Solving tactics. Lichess has a very nice platform with hundreands od thousants of puzzles that occured in real games. Devote some time solving them, and then check out the analysis provided by the computer. You may discover patterns and combinations to use in your own games.
2) Learn some opening theory. It is very useful to know the ideas of an opening, potentials, weaknesses, possible tactics and any stuff like that. Find an opening for every color, use it until you know how to play until move 10 or somewhere around there and you will save time for the middlegame and decreased the possibility of being caught off-guard. Not to mention that you might trap your opponent as well. Then you could move to another opening or perfect your already working knowledge of that one.
3) Study the endgame. This is the most difficult part of the game. Many games are defined by the endgame. You will be on an advantageous position if you know how to treat endgames
4) Check games played by masters. Find a playstyle and check games of a world champion with the same playstyle. Even adopt the openings they used. If it worked for them, it might work for you.
So this is what I have to say. Of course, on top of all of them, you should play longer games to practise everything mentioned above. You will see that quite soon you wil improve.