Left click to check for a mine, right click to place a flag. You can also just click the flag icon at the top of the screen to switch between normal and flag placing mode. If you are using touchscreen then that is the only way to place flags.
This is a clone of the classic minesweeper game by Microsoft. Search for mines by clicking each space. The numbers each space show after clicking them indicate how many mines are currently touching that space. Try to clear all the spaces that are not mines. To start your current game over just press the smiley face button