What makes up a good forum/story/fanfiction?

Its great that you want to write a fanfic on this amazing site.

How to be popular?
Thats a tough question. It takes a lot of work to be a popular as Ferb!

Thou shall write a good fanfic.
Writing can always be improved every time you rewrite it (Even if it means you won't be able to move your fingers for a week). Big mistake number one: NEVER, EVER write your fanfic and post that ever chapter without editing it. It makes you look sloppy and lazy. I hope you are not.

Big mistake number two: Post the chapter right after you have writen it. That make the fanfic look, again, sloppy. Write, edit, write, edit, write, edit, write, and now post (You don't have to edit more then two times if you want).
Popularity is also making friends. We are a nice bunch of people from where-ever talking about a grrreat series. Comment on fanfics, articles, and questions to make friends (Props and adding them to your fanlist is nice).

Thats how to have a popular fanfic young grasshopper. And remember to follow the rules.

Well, I have been a fanfiction.net writer for quite a while and I have been on fanpop longer than any of oldest people on this spot (I'm serious).
And I've learned that to make your story POPULAR, you just have to be really a friendly person for real and people will WANT to read your stories just because it is yours.
Also, you have to write it well. I agree with the person above me that you should always write and edit. You must edit. But when you write long enough, and your confident enough, you can just edit as you write, which will give your story the originality it needs. Usually when we linger on our stories for too long we are influenced by the last story we read, hence making it slightly less original.
Which brings us to originality. It's cool to collect ideas from other places and add them in your story here and there, but people will REALLY love your story if it is YOURS. If it is original. Making it a slight AU (Alternate Universe - a term used for fan fictions that have twisted the facts and original story by the original author) fan fic usually helps with that. Most people like Slight AU's.
And now of course for the grammar and punctuation. Always get that right. THAT is the main reason why we authors edit and re-read. To see for any spelling/ grammar/ punctuation mistake. Usually Microsoft word will do that bit for you but, always stay on the safe side. And proper English is no joke. People HATE fan fics with improper English.
Another important pointer is that your story has to be tied together by a plot. Not just here there writing whatever comes to your mind. Come up with a background and base for your story, then fill in the spaces in-between. That is the only way to keep your fiction organized.
Last but not least - especially when you are dealing with Mythology - get your facts right. Do your research h well because one story in mythology has twenty different faces. Always use the most common version of the story you need so that arguments do not arise. If the version that suits your fiction best is not the common one, please point that out in an authors note. You should have an authors note at the beginning and end of every chapter, just to let the readers in on some basics that they should know before they read your chapter.

Here are some more minor pointers:
- Always have a good, gripping summary that ends with a "..." so that the readers will WANT to read on.
- Your first chapter must be interesting otherwise your readers will not be willing to read forward.
- Never take harsh criticism badly, and address it with exception that you may have this flaw in you. It will only help you, even if the critique is just trying to be plain mean.
- Have interesting names for your OC's and make up interesting personalities to keep th readers hooked with characters they have never known.

And that, my friend, is what it takes to write a good fan fiction :)

Good Luck!