Great start!
First off, your actual characters in the shot are pretty well drawn. You have a good start to a style, and that style will improve as you practice more.
Your zombie crowd is much better drawn than the bloke in the laundromat. The way the guy in the blue shirt is leaning doesn't convey much force. If you drew a line going down his spine and continue the line naturally in both directions, you'll notice that your person is leaning backwards. That's not a very effective way to block a door; he'll just topple over! Try drawing a few pictures of
just that guy holding the door shut a different way. Try putting his legs farther back and away from the door and making him hold the door shut with his shoulders. That will make him look like he's putting more weight behind him to hold the door shut.
Your main issue, as many have said, is your use of perspective; it's just not quite there. The angle of the street doesn't match up with the horizon or the size of the buildings in the background, and the tiling and perspective in the laundromat just feel off. For example, those tiles should be getting bigger as they get closer to you. Because of the way your perspectives line up, the guy in the green shirt looks a lot smaller than the other guy (assuming they were both standing). Even if he's supposed to be a little shorter, you should make him look just a bit bigger since he's closer to the viewer.
Also, your area outside needs some tweaking. The skyscrapers look like they're all facing different directions, but they should all be facing the same direction in a city (take a look at a satellite map of any big city on Google; the buildings will all be facing the same way).
On a final note, you should take a look at the road you drew. One car should be able to fit in one lane of the street and have some extra space on each side. Either everyone drives tiny cars in your world, or those are some huge zombies!
Overall, you just need some more practice. You're off to a great start, so don't give up!

If you want me to explain any other parts of your picture that could use some improvement, just send me a PM and we can arrange to talk on Steam, MSN, or something like that.