The 1-2 pattern
A 1 beside a 2 along a wall: the far cell past the 2 is always a mine.
A 1 next to a 2 along a wall is the most useful shape in the game. The 2 wants one more mine than the 1 does and sees exactly one cell the 1 cannot, so that cell is a mine every time.
Drill 1 of 4
Flag every mine and open every safe cell you can prove.