Try to expose every square in the grid that is not one of the hidden mines, without opening any square that is a mine.
Click in a square to open it. Every opened square are marked with the number of mines in the surrounding 8 squares, if there are any; if not, all the surrounding squares are automatically opened.
Right-click in a square to mark it with a flag if you think it is a mine. If a numbered square has exactly the right number of flags around it, you can click in it to open all the squares around it that are not flagged.
The first square you open is guaranteed to be safe, and (by default) you are guaranteed to be able to solve the whole grid by deduction rather than guesswork. (Deductions may require you to think about the total number of mines.)