The object of Minesweeper is to uncover every square that does not contain a mine.
When you uncover a square, you can see how many mines surround that square. You can use those numbers to determine where the mines are.
If you think a square has a mine, you can turn the square red to show that. You can also turn the square yellow if you think the square has a mine, but you're not sure.
To start the game, click any square. The computer will make sure that this square doesn't have a mine, unless you've turned that option off.
To uncover a mine, click it. If there are no mines surrounding the square, the computer will automatically uncover those squares as well. If the square has a mine in it, you lose, unless mistakes are allowed.
To mark a square as containing a mine, hold down Shift while clicking the square. If your mouse has two buttons, you can also use the right mouse button.
To mark a square as maybe containing a mine, shift-click it again. Shift-click it a third time to restore the normal state.