What the title says. It's good that the Fanfic section's getting a lot more popular now, but the new writers need to use good English to post. I'll list out a few essentials first.

-Punctuation. [IMPORTANT]
-Spelling
-Paragraphing
-Get your tenses right!
-Descriptions (both character AND environmental) and elaborations can make up 10% or more of a story. [IMPORTANT]

Less important ones would be:

-Dialogue helps!
-Don't make your story look like a summary!
-Try not to get the royal "WALL OF TEXT" comment that usually results from lack of paragraphing. (Sarcasm FTW)
-Try to maintain some sense of originality instead of ripping off anime or some other TV show.
-Keep the story going smoothly. Don't make it look way too abrupt.

Writer/Reader-submitted:

Kon:
-Also, remember that people aren't forced to read your story. Try to make it somewhat attractive, don't steal too many ideas or rewrite something from a TV series or a game.
-Listening to music or doing something completely different can help beating that evil writer's block.
Xorten:
-Just put in the title, saves having to read massive titles or titles that don't have anything to do with the story.
Starlit Azure:
-Avoid using CAPS. Exclamation points work equally as well and don't give a headache.
(Kira's comment: Of course, you could use both to make a situation similar to yelling into a megaphone)
NegiWarrior:
-Don't hit enter after every line!
xdgr:
-A side document with all your character equipments and stuff will help if you forget things and keep it more organized.
jiahao_94:
-If you want to link a past item to a present event, flashbacks/dialogs are useful tools.