When you get off the bus at the start, the two cars surrounding the bus have the triangle button pointing to them to climb in. However, they do not point to the bus you rode in. That bus is practically indestructible. It also destroys other cars in one hit. Watch out for the helicopters dropping bombs as they are the only thing that can destroy the bus.
After you fight the Boss (giant green robot with the eye and a fork and knife) and his life reaches zero, he will start exploding. Keep shooting at his eye. He will just continue exploding as long as you shoot, and you will keep gaining points. You can stop when you have about 60,000 points. At the end of the level, you will gain about seventeen lives.