Start by going to the Olympic Games and enter Olympic Mode. Select the Javelin event on Medium difficulty. Follow along through the tutorials until you get to the actual event.
To run down the track, alternate pressing the "A" and "B" buttons as fast as you can. The more speed you can get, the better your throw will be. Before reaching the end of the track hold down the "LB" button and release it when it hits the green area of the gauge. Now use "LS" to steer your javelin as it flies. Guide the javelin through as many clouds as possible. The more clouds you can hit, the higher you score will be.