The easiest way to score a goal is to push the D-Pad on the opposite direction when shooting. Doing this, you will get a "High Shot" and most of the times, depending mainly on your position on the field, the goalkeeper will not get it and the ball will go straight into the net. It's pretty tricky, but you can also choose the side of the goal you want your shot to go into, this way (assume you're using the "Side" camera, and attacking to the right side of the screen.

D-Pad position
Left
Left + Up
Left + Down

Shot direction
Top (center)
Top Left corner
Top Right corner