I'm not going to talk about spelling mistakes (shame on you, with spell check?) Even if it's the wrong word, at least if it's spelled right, you get 1/2 a point. The absolute correct format style isn't so absolute with the exception of double-spaced, typed (yeah, I have to put that in), and 12 point New York Times or Courier font. Where new chapters start (ideally halfway down the page) is sometimes moved to the top, 2/3 down, and places in between. And if you're bad at contractions (Really? At this age? Get a book on grammar!), at least you know it and should make an attempt to correct as many mistakes as possible.
My favorite pet peeves are 1) use of the 'helping' verb and 2) sloppy editing. First, what 'help' is a word that waters down action and drags the story out? You want dynamic action verbs. To wit:
I had started to run down the street when the zombies suddenly appeared, armed with chainsaws.
Is that better than:
I ran down the street when the zombies suddenly appeared, armed with chainsaws.
Did I really need to use 'started' to run (because I did, didn't I?) and if it's past tense, did I need the 'had'? Answer to both is no. And, you saved three words: had, started and to on the infinitive. Plus, doesn't it sound more exciting? You're cutting right to the action. Go through your manuscript (do a global search, it's easier) for had and have and see how many times you used it. Some you have to keep, (I can use 'must' here and get rid of 'have to') but most you can eliminate.
Sloppy editing is the bane of my writing life. We all slide a manuscript by from time to time, thinking our friends will fix it up. Besides not being fair to your friends asking them to do your work, you won't sharpen your own skills. Your friends won't be available to continually edit your writing. And we all know that sloppy editing will kill your story, even if the editor or agent thinks it has a lot of potential. They don't want to do all that work, and if you can't be bothered, why should they? There are thousands of writers out there with polished manuscripts, so it's easier, quicker, and cheaper to select someone who puts every effort into polishing their text.
A few mistakes are no biggie. We've all seen mistakes in the final book (although I have to say I haven't found any in Sirenz because Sandy Sullivan, our Flux copyeditor, is a goddess of editing. But if you find any, let me know and not only will I reward you, but I'll have it fixed for the next printing.).
The way to perfect your editing--numerous read throughs. You can't get all the mistakes the first pass through, because you'll make changes and that will lead to new mistakes. After you've read it no less than twice (stop groaning, this is the real world), pass it to a friend. Make changes, read through. My secret? When Natalie and I were writing/revising/editing Sirenz, we read it aloud, and we read the other's chapters. If I stumbled on a sentence, or had to reread it to understand it, we knew she had to fix that sentence. Reading aloud helps you to actually hear how your work sounds. It works even better with someone who doesn't know the story because they have fresh eyes, and won't unconsciously skim over parts because they're too familiar with the text. Time consuming? Yes. But when our editor Brian Farrey read Sirenz, besides telling us we were funny :) he said it was a polished work. I have no doubt that this helped in his decision to sign us even though we had to trash 15 out of 20 chapters. The writing would be there, but in a modified version.
Now go write, and write well!