You have to pass by the pit lane in the shortest possible time. Run, stop in the pits, change tires, refuel and go to the final line as fast as possible!
Keep pressed the mouse button to accelerate and release it to stop. When you see an arrow on the way it means that the pits are close, just release the mouse button to park close to the pit lane. Then click at the red rectangles while you see some wheels and car parts appear. When you're ready, click again to run to the end.