Only move to the edge of the square you have to click, not its center. Avoid back-and-forth movements by organizing 3 or 4 clicks into a line. A move that doesn't open a square or mark a mine is a mistake. An unnecessary movement of the mouse is a mistake.
If the square harbors a mine, the player loses. If instead a number (from 1 to 8) appears on the square, it specifies how many mines are present in the eight squares surrounding the numbered square. It's clear, for example, that finding a square numbered 8 means that all eight adjacent squares contain mines.