Hey, I got credit for something and I wasn't even here! I do agree with everything Iggy said. Know the characters and story before you start writing about them. Also, write a story idea that interests you.
Personally, I would love to do a CloudXReno fic, but there are so many out there, I think I'd have a hard time coming up with something original. I especially love Iggy's Strange Bedfellows fic, It was brilliant. The author Squishy has some really great, hot RenoXCloud fics as well.
I really don't have many tips to give you, I've only been writing fics for a little under a year now. Just do your best and see where it goes. That's what I did and I get great responses for my fiction.