I'm a simple fan of sci-fi and fantasy, with a bit of an interest in anime.
Things that will help you succeed in writing fanfiction:
1. Have the ability to write well in your language of choice. If you're reading this, it's probably English.
2. Have your story planned out for at least the first few chapters. Have a long-term goal for the fanfic in mind.
3. Have a well written summary that will catch the eyes of potential readers. If it's your first fanfic, do NOT include that fact in your summary.
4. Don't take constructive criticism negatively. Understand that other people want you to succeed, and may have ideas to help you.
5. Keep working at it. Motivation is hard to sustain when real life has its own problems and distractions, but stopping is always hard to come back from.