1. Do your research before you start writing
How do certain characters act? What should you and should not do? Why does a particular thing act that way? All very lovely questions. Lovely questions that could potentially rip that fanfic your considering writing down in two. So, in order to avoid your dreams being crushed by canon, do some research! Ask your self questions about canon and don't be afraid to take out any unnessesary details in your story. Things like filler, lifeless plots and terrible romances that serve nothing more than to quench the viever's thirst of seeing the paired couple.
2. Take constructive criticism
Espicially if your a new writer wanting to start writing. Sure, you worked hard on that fighting scene and that brilliant main character, but still, there are going to be things that you might get wrong. Don't worry about that, silly human! Just try and come back to the story if you feel like you need a break, and magically you can see all the things that you fucked up on! Yay! Now, go forth and clean up your mess, human, before you further embarrass yourself!
3. Edit your story
Once your done writing your fanfic, read it and check for grammar mistakes and typos. If you're not bothered to check your own story for small things such as using "you're" instead of "your", then why should anyone else take interest in it?
4. Write down your ideas
We've all been through the highly inconvenient time where you once had an absolutely amazing idea during the middle of the night. You tell yourself to go and do something with it in the morning, thinking about all the lovely praise senpai will give you until...you forget your idea. Brilliant. You can always prevent this problem by writing it down somewhere, but this also applies to other things. If you like a certain character in a TV show, think about what makes that character interesting to you and write it down somewhere. Who knowns? Perhaps this may just be the start of a truly wonderful idea/character...speaking of which...
I might expand up on this later, if I get enough feedback off of it. Please let me know what you think in the comments! Thanks for reading! :D