Pick a game against one of the big time players.

The best way to learn is to watch your opponent and see what works, or while in practice mode click to watch best goals submitted by other players.

There are two basic ways to take your kick. #1. just move mouse to aim and then click and release on either side of the ball, or directly on it for a straight kick. Click slightly below the ball to aim higher.
#2. To curve your kicks; first aim, then click but hold left mouse button then drag to the opposite side of the ball and/or up. Release left mouse button when ready

Controls are pretty sensitive so you do not need to click far below the ball to make the ball go up, but if you do and then release while dragging mouse cursor high, it will make for some very interesting free kicks.