We all makes mistakes when we write, it happens, can't be avoided. however, there are some things to be watching out for when you write a story.

First off: Check your spelling, grammar and punctuation. You would not believe how many people get turned off when they see a story that opens like this:

i looked in the room too see if my lamp was still on. sure nuff they're it sat burning bright.

How horrible is that?

I looked in the room to see if the lamp was still on. Sure enough, there it sat, burning brightly.

Better, isn't it?

Spelling: To err is human but when you're misspelling every other word, you got problems. Use spell-check on Microsoft Word or Google has some pretty awesome writing utilities, including said spell-check. Use it!

Grammar: Not always necessary but it never hurts, sometimes it's nice to read things that are written proper. Unless of course your story CALLS for bad grammar, than have it! Now, for those of you who confuse your meanings, we'll have a separate piece for you, eventually. It'll be for everyone who doesn't know whether to use they're, their or there

Punctuation: Commas, colons, capital letters, all of it. Use them. A comma can be over used (Just ask Ly) but at the same time, it indicates a pause or a break in a sentence. 

Secondly: When you actually write your story always remember that while there's nothing wrong with using a 'Mary Sue' type character, kindly refrain from making said character damn near invincible. Whether you're writing an original story, fan fiction, whatever; it becomes boring when the main character kicks everyone's ass and can't be touched.

One more thing: When you're thinking up your character, make sure you make the person realistic.

Blackheart Writing Commandments
Thou shalt remember to use your spell-check.
Thou shalt not over use the comma (LY!).
Thou shall remember that your character is supposed to be a person, not a super hero (unless the story is one of them kind of stories).