I never thought I’d have to do a post like this, but it turns out more people than I ever thought don’t know these things!  Here are the basics to keep in mind while you write.  You can deviate from them if you wish, but it would be hard to write a good story without even knowing about these things.

1. You need to know proper punctuation and grammar.  Putting commas where your periods should be is not a stylistic choice.  It’s not a good one, anyhow.  There are several websites that explain grammar and punctuation in simple terms.  If you’re confused about something, look it up.  There’s no shame in doing so.  I do it all the time.
2. The fewer words you use, the better.  The point is usually to tell a good story, not to have a high word count.  If something is extraneous, cut it out, even if it sounds really good.
3. Don’t show off your vocabulary.  Readers are not impressed by big words where smaller ones can do the job.
4. When attributing dialogues, said and asked are your friends.  Never say “he commanded” when the words themselves are commanding.  As far as I’ve seen, beginning authors think this is a matter of style while professionals know not to clutter their dialogues with alternate words for said.
5. Speech doesn’t have to conform perfectly to grammatical rules.  In fact, it probably shouldn’t.  Sentence fragments add flavor and people often use words in new and technically incorrect ways.  This isn’t to say you should make dialogues entirely true to the way people talk in life.  That would be hard to read.  You should make them far less formal than the narrative, though.  Each character needs a distinct voice.  This not only adds realism, it also makes it easier for readers to tell who’s talking without overt clues.
6. Your beginning sentences should make the reader want to know more.  
7. You should not bring new items, characters, places, or situations in late in the story without any foreshadowing.
8. Suddenly is a horrible word that should be used as little as possible.