Clear every safe cell. Numbers tell you how many mines are touching that cell.
Tap a cell to reveal it. If it's a mine, the round ends — but your very first tap is always guaranteed safe, so you'll never lose on move one. A revealed number tells you exactly how many mines are hidden in the up-to-eight cells touching it. Use that information to work out which nearby cells are safe and which hide mines.
Right-click on desktop, or turn on Flag Mode and tap on mobile, to mark a cell you're sure hides a mine — this stops you from accidentally revealing it and helps you keep track. You win by revealing every single safe cell; you don't need to flag every mine to win.