Your job is to protect the police van and you need to shoot al the thugs that are following the van. So shoot them as fast as you see them. If you run out of ammunition you just need to press the space bar to reload. Each level gives you corresponding points and money.
You can use the money to upgrade your weapon and buy some military back-up. Buy them as soon as they become available because the levels keep getting hard after the first one. Once you bought the most powerful weapon then make sure that no one comes near the van.