This idea is going to sound obvious, but tennis is supposed to be basic.
If you are serving, focus on getting the first serve in, then hitting a shot or two (usually crosscourt) to get the opponent moving, and then figure out a way to win the point. When returning serve, get the return in play, hit a shot or two (usually crosscourt) to get the opponent moving, and then figure out a way to win the point.
To expand on the last part (figure out a way to win the point), usually hitting your shot to the opening, adding a little speed or depth to the shot, or moving forward as you hit your shot (hitting the ball early), will be enough to win the point. You might have to do one of these things on a couple of shots in a row against the better players at the end of the point.
In summary, get the point started, get the opponent moving, and look for a shot you can hit to win the point. Keep your strategy this simple until you are consistently executing it. The player who does these things best throughout the match is almost always the winner.