1. Thou shall not take credit for work that does not belong to them.
One of the most important rules of fanfiction writing, in my opinion: Credit the original creator. Always have a disclaimer somewhere amongst your works, whether it be each individual chapter or just a simple "I own nothing" statement on your profile. I, being a lazy ass, use the latter choice of that sentence. If you make the decision to use song lyrics for something, credit the artist as well. Same with fanart--the characters you are drawing are not yours. I hate to break it to you, but you don't own Rainbow Dash. The Sonic Screwdriver did not pop up first in your overactive imagination. A certain blonde ninja obsessed with ramen who has a nine-tailed fox demon sealed inside of him is not your creation.
2. Thou shall enlist the help of a dictionary, thesaurus, encyclopedia, spellcheck, and an editor before submitting their work.
Another biggie for me. I take great pains to make sure my pieces--fanfiction or not--are as perfect as I can get them to be. Yes, I know I am not infallible, but at least I try. It irritates me to no end when I see something where every third word is spelled wrong; when certain words are used over and over again in repetition when you could find a more intricate phrase to replace it; and when there are so many grammatical errors that it would make my language teacher's eyes bleed. Please, please, please try to always edit and look over your work as many times as you can until nearly all errors are erased.
3. Thou shall not write in one block of text.
A very big pet peeve of mine. Separate your paragraphs, people. Whenever someone begins speaking, begin a new paragraph. Whenever the scene or mood changes, begin a new paragraph. For the love of every deity to ever have a prayer uttered to them, do not just slap 1000+ words in one big wall of words without separation and without spacing out sentences or quotation marks. It literally hurts to read.
4. Thou shall not make characters OOC.
OOC, or "out-of-character." Something that is very prominent in many fanfictions that lie within the internet. Here's an example (let's use the Avengers for this, since 90% of the people who read this most likely will share my love for that movie): Loki is a god. An evil god. An evil demigod frost giant person with an ego the size of Antarctica and the desire to enslave humanity and become emperor of the universe. He will not, in any scenario, fall in love with a regular human girl in the span of two seconds. He will most likely kidnap the poor thing and torment her until she goes insane if he actually decides to bother with her. If you decide to pop in your own little OC as Loki's love interest, make the relationship seem believable in both the timespan and actual feelings that grow.
5. Thou shall not pretend to be fluent in Google translate.
Let's face it, people. We're not all bilingual. At most, I bet a lot of the people reading this know the language they were raised with and a different language they learned. Google translate is okay for a couple of words or phrases here or there--I use it in that way. But if you decide to write an entire dialogue using only Google translate, chances are you're going to get a lot of words wrong. Google translate tends to be correct only three out of five times, in my opinion; therefore, use another website as a resource or even a person fluent in the language you want to use. And yes, Hetalia fans, I'm talking to you. Just because the countries can speak whatever language that was born in them does not mean you can as well.
6. Thou shall never beg for feedback.
"OH MY FUCKING GOD PLEASE REVIEW PLEASE TELL ME HOW PERFECT THIS IS I WORKED SO FUCKING HARD ON IT I DESERVE IT!" Seem familiar? Yes, that's a bit of an exaggeration, but here's the truth: begging for reviews, comments, critiques, favorites, etc. will really get you nowhere. People don't like to feel pressured when trying to decide whether or not the story they just read deserves their feedback. Asking politely, such as, "Feedback is appreciated!" is a more credible approach. Do you think J.K. Rowling went to Bloomsbury demanding in all caps "PUBLISH MY MANUSCRIPT"? No. If she could be patient (yet anxious) and await her works to get reviewed, then you can too. It's not too hard.
7. Thou shall not flame.
What gets writers thinking their work is absolute shit is when someone point-blank comes out and says it in as rude a manner as they can. Yes, maybe their writing needs a little more work and elbow grease--but rather than telling them it's awful, why not let them know what they can improve on and what they're doing correctly? And doing it politely. It's never good to start a bashing match. It shows just how immature you are.
8. Thou shall not react violently to criticism.
This goes with the above commandment. If you are the person receiving the criticism or possible flaming, don't lash out. It shows you are immature as well, as well as giving the flamer the negative reaction they most likely wanted. Either ignore it if it's a blatant "This sucks" remark, or kindly thank the person who posted the review if it is constructive criticism telling you what you can improve on. Everyone can always grow in their writing, whether they're George R. R. Martin or a teenager with a secret Pokémon fanfiction itch.
9. Thou shall not plagiarize.
This goes for writing everywhere: fanfiction, original works, essays, projects, etc. It literally sickens me when I see someone who has completely stolen another person's hard work and claimed it as their own. If you plagiarize something in real life--especially college--I guarantee you will face the consequences. It can be as simple as being banned from a site, to as critical as having your semester final essay being shoved into a paper shredded to become gerbil litter.
10. Thou shall always have fun.
Writing is about what you want to do, not others. Fanfiction is definitely one of the things where you're allowed to go wild--as long as you keep it within believable limits. Be creative, but always stay true to the characters' personalities or the worlds in which they live in. If writing fanfiction isn't fun, then why are you still doing it? Don't feel forced and try to keep going just for the sake of trying to please the masses--believe me, I've learned from experience. Just remember; you're the one holding the pen. It's up to you to decide who lives, who dies, who loves, who cries, and who gets attacked by radioactive sporks while on vacation in Botswana.