Know the enemy
You don't need to play well in order to win a chess game. It is enough to play better than your opponent. In fact, it's usually enough to play better than your opponent at just one key point in the game. That point occurs when one player secures a decisive advantage that leads to a win. The trick is to realise when the game is approaching a critical point and to seize the opportunity.
As for deciding on the next move, you don't need to find the very best move in the current position. You only need to find a move that works against this opponent at this time. As Jonathan Rowson points out, chess is not a logical puzzle but a battle between two minds. It is not an objective problem but an inter-subjective contest between two people. It is a contest between two fallible and creative subjects, not between two methodical calculators. Your opponent has hopes and fears, just as you do. Some moves have a psychological impact, eg a pawn advance, check or sacrifice that is less dangerous than it looks.
If you play someone every week in tennis or squash then you get to know their style, their strengths and weaknesses, and the sort of shots they like to play. The same goes for a chess partner. Knowing their habits and patterns gives you an advantage compared to playing them for the first time. Even if you have never played a person on chess.com before you can look up their record. The most obvious bit of data is their rating, but you can also see how much they have played, against whom, and with what result.
How many games they play at once is a key factor to consider - the more they play simultaneously the less likely they are to play carefully. I once played a Romanian who was engaged in some 300 simultaneous games. I knew he had little time for careful analysis and sure enough, he soon blundered. If your opponent's blitz rating is much lower than their online one then they are probably an older player, and vice versa. If you are really serious then you can even play through their past games. This is what masters do when they prepare for tournaments.
If the opponent plays a strange move or irregular opening then either they don't know what they are doing or else they may want to experiment or simply vary. It's best to assume that they should be taken seriously until shown otherwise. By the time you reach the late middle game you have some idea of your opponent. Are they impulsive or considered, cautious or adventurous, flexible or not, error-prone or accurate, weak or strong? You know whether they attack at every opportunity or prefer careful defence. You might have an idea of whether they like to swap pieces, whether they prefer Bishop to Knight, whether they prefer simplicity or complexity, seek or avoid the endgame, what openings they favour, and which side they castle on. Are they likely to resort to desperate measures for a win, or will they settle for a draw?
Once you have an idea of their strengths, weaknesses and habitual mode of play, you can take these into account when plotting your moves. It is like knowing your enemy in a war - knowledge can be invaluable.
