To shoot, tap anywhere on the screen, hold, and release. The timing of tapping and releasing is the key. There is a target moving around the goal. Tap anywhere on the screen to stop the target. If you stop it outside of the goal - you miss. If you don't stop it before the timer on the scoreboard ends - you will automatically miss. Make sure to hold down when pressing to stop the target, because the precision bar will appear. Try to release your finger when the release bar is in the green area for as much precision on your shot as possible.

The precision bar has 3 different colors:
Green: Perfect precision - you will hit the target.
Blue: Good precision - you will sometimes hit the target. The closer to the green you stop it, the higher the chances of hitting it.
Transparent: Bad precision - you will miss the goal.

To defend, drag the slider anywhere on the screen and move the blue area left or right. By moving the blue area, you control your goalkeeper. The blue area is entirely safe; the goalkeeper will defend the shot if it goes there.
You need to select the area to defend before the opponent shoots, so be quick when defending.