Click on the play button. On the next screen, select the first level (or higher if this isn't your first time playing the game).
Each level will have its own challenge, usually to get the ball in the hole in the least amount of swings.
To hit the ball, use your mouse or finger to click on the ball and drag your mouse cursor or finger back to create a white arrow. The longer the arrow (pulling your finger or mouse back more) the stronger you'll hit the ball. You can of course change the angle as well.