A warm breeze blew across the desert landscape, kicking up dust and decaying chunks of paper. It was a hot day on this plain but the heat never bothered the wondering dead as they aimlessly crowded an abandoned freeway gas station. Roughly fifteen of them either stood still, bumped into broken down vehicles, meandered and/or leaned against the wall; each of these things where hunting for a meal but none of their senses could pick nothing up. However, they all came to action when they herd a sudden PUNK! as something slammed into the colorless metal sign.
Your first sentence is strong, rapidly setting the scene with good imagery and movement.
The first part of the second sentence undoes the work of the first - you already established that it was hot on the plain. The rest of the sentence is good, but it piles adjectives together (which can work, but is usually bad form). Better: "The heat never bothered the wandering dead as they crowded an abandoned gas station by the freeway."
The third sentence also doesn't give new information. One may argue that it does give us a number (about fifteen) but even here the number is unnecessary, since we can already about that number when we think of zombies crowding around in a gas station. Although some authors like to restate things for the sake of the poetry, this sentence reports what's happening matter-of-factly and thus merely slows the progression of the story.
The fourth sentence is better, but it starts off with "however," which is usually not a good word for narrative passages and doesn't work well here. Also, I've come across authors (like Jim Butcher) who say that action should be reported in terms of stimulus -> response
. So you might try "Something slammed into the colorless metal sign, breaking the silence with a sudden PUNK!" and then have the zombies jerk into motion in the following paragraph.
This sound repeated again three times before the undead were surrounding the sign above them, drawn out by the noise. Among the fifteencomma one of them had a pair of glowing red eyesperiod It turned its gaze upward to glare at the signperiod Perfect. A minute passed, then a whiz followed by a sudden fleshy impact, and the red eyed zombie’s head snapped forward. The zombie fell face down with a lifeless thud. A split second latercomma a rifle report was heard. None of the zombies stirred from the sign **for the sound was still too distant from their sensitive ears.
Mostly grammatical errors here, which I've corrected and bolded. You've also got passive voice where I underlined it; you can get away with it, but it's usually better to pick a stronger verb and reword it. Also, check the asterisks in the last sentence - it's usually better to allow the reader to make inferences to begin with without explaining in terms of "X, for Y..." In addition, you just told us that the rifle report "was heard" and then say "the sound was too distant from their sensitive ears." So I'd just cut everything after "the sign."
Reading further in, it looks like you have good action, and the ability to generate questions that draw the reader in. (Why is the shooter here? What is the calm voice from nowhere?) And you have a rudimentary sense of the dramatic that really helps. But, there are a lot of spelling and grammatical errors that make me flinch. A spell & grammar checker should fix most of the problems.
Hope that helps!