Answer:
You will need:
1. Some posts + bar = Goal
2. Some sort of barricade behind the goal to stop the ball from flying everywhere!
3. A figure to be the goalkeeper
4. A marble or something small, round and easy to hit!
5. A small (or large it doesn't really matter) Lego mat to put all of this onto!
Then once you have built all of this, place the ball/marble onto the Lego mat so it stays in place. Place the ball where you want to; it gets harder if you place the ball really close or really far away. Place the ball at different angles to expand the game into free kick mode as well as penalty mode. Now off you go and have some fun!