GET DOWN TO THE BASICS!
You may look at the heading above and be compelled to just skip this chapter thinking, 'pshhhh, I already know this!' but I'll be darned if you don't learn something, or at least refresh your mind.
First things first.
The dreaded….SPELLING AND GRAMMAR! AAAHHHHHHHHHHHH
I cannot stress enough just how important it is to not only spell words correctly, but to use them properly. For example, I am in the middle of reading a fanfiction right now, and I absolutely adore this author's plot and character development (to be discussed in further chapters), but I am driven INSANE by their spelling and misuse of words. And when I read their profiles, they claim to be in sophomore year of high school or older. IF YOU DON'T KNOW HOW TO PROPERLY USE 'THEIR', 'THEY'RE', OR 'THERE' BY HIGH SCHOOL, YOU'VE GOT ANOTHER THING COMING FOR YA. To review:
THEIR: shows possession to a plural group of nouns. ex: THEIR dog is very cute.
THEY'RE: combines the two words 'they' and 'are' together (if you don't know what those words mean, you shouldn't be writing fanfiction): so in short, if you wouldn't put 'they are' where you put 'they're' you SHOULD'NT USE IT! ex: THEY'RE going to go crazy because of how cute that dog is.
THERE: refers to a specific place, usually where a specific object is. ex: They skipped over THERE.
Those are only the most common words, but there are many more homonyms such as pair and pear, plain and plane, course and coarse, ant and aunt, bore and boar, you're and your, brake and break, etc.
TIP: if you type your stories on a computer, USE SPELLCHECK! It's there for a reason! No one wants to read a fanfiction that has too many spelling errors.
NOTE: if you feel like you use the same word to much, like you say ' dark ' excessively, use that handy-dandy right click (on PC's) while highlighting the word, and hover over 'synonyms', it'll give you words with the same meanings, like dim, shadowy, shady, gloomy, murky, sinister, and evil. BUT BEWARE! Make sure you know what you want the word to mean, because dark like black and dark like evil are completely different.
And now….SENTENCE STRUCTURE! Eeeeww…..
You, and, I, both, know, that, your, sentences, should, not, look, like, this. If you find that you hit the comma button more than 5 FRAKING TIMES in a sentence (unless you're listing something)then you may want to read over your sentence and see if you can slit up the sentence into two, without making a fragment or phrase (you know when you're on spell check and it says Fragment {consider revising}? Yeah, that)
TIP: If you have trouble deciding where to put a comma, try reading the sentence out loud, and see where you naturally pause while speaking
On a different note, if you find you're making a really long sentence, you may be creating something we Americans (and I'm pretty sure everyone else, correct me if I'm wrong) call a run-on sentence. The only time you should be creating one of these, is if you are trying to write dialogue for nudge and you feel it will help develop character. If you are unfamiliar with run-on sentences they look something like this because they have little to no commas and they usually have the word and in here more than one time and they don't sound natural and you feel weird typing it.
Going back to the fragments and phrases, A good thing to keep in mind is that you're sentence should have a complete subject (noun) and a predicate (the verb, adjectives, anything BUT the noun)
Hey, at least it's me telling you this stuff and not you're teacher, yeah, you know the one that spits a lot when they're talking? Yeah, that one.