The zombies are hungry and they won't stop at nothing just to eat. When they saw your house, they learned that there are many food in it. You need to protect your food or they would eat all of it and there would be nothing left for you to munch on. Fortunately, Osama told you about it, so you can defend your house from the zombies. However, he don't anything for free.
The idea here is to kill all zombies before they can even get close to the house. Zombies will storm your house, but you won't have a gun to protect your house at the start. You would need to use stones to defeat the zombies. Each kill would earn you money and you can use the money to buy weapons. Don't worry about those zombies as you would get help from Rambo at the end of the first level. After the first level, you already have money and you can some weapon to help you with. Use it to defend the house from attacking zombies next time. The game gets harder and harder as you make some progress, so it is better to buy better weapons to protect the house with.