I have had this really good idea for a fan fiction in my head for awhile now. But every time I go to write i just can't do it. I keep getting distracted by other things and I just need advice on how to stay focused.
Hmm, each person is different so any advice offered may not work, mind, but one way to approach it is to combat the stuff distracting you specifically. For instance if it's the internet in general - switch it off so you won't be so tempted to go visit that other website instead of writing, and so forth. Sometimes just starting is the hard part too - once you get going continuing comes easier, so just forcing yourself to get something down could help, even if it's say on paper. Sometimes music helps people write as well.
There's also programs you can google such as Write or Die, which although costs money has a free web app - the general gist is that it'll bug you if you stop writing. You can also get people to bug you via requesting a deadline in the fanfic club (check the clubs section) or getting a friend to remind you too.
If the problem is more that you are unsure how to start the story itself, then try writing the general gist of the story plotwise or writing out what the characters are, etc - exploring that can help you get started with the actual story.
I made a post about this about 2 or 3 weeks ago. There was some good advice in there, and it should still be on page 2.
The best thing I can tell you is to schedule time to write. Look at your upcoming week and schedule yourself. I don't know how old you are or where you are in life, but let's say it looks like this:
6am: wake up, get ready for school/work
7am - 5pm: school/work (including travel)
5pm - 5:30: unwind and relax
5:30pm-6:30: dinner (including preparation if needed)
6:30pm-8: random tasks (homework/housework, etc)
9:30pm-10: get ready for bed
That's just a sample for one day, but try scheduling yourself ahead of time so that you can look forward to your writing.