Have you thought about starting off writing just one shots? Just as a start... sometimes they morph themselves into chaptered stories.
As for motivation... I find a clear plot outline and knowing the ending of my story before I start writing often helps, and then to start posting and getting reviews keeps me motivated too. Then I have a few friends in MSN who like to kick my bottom if I don't post, and they help me slog out a tricky area or a gap in my plot.
Make sure you really like your story. This isn't conceited... you wouldn't keep
reading a story you don't like, so why would you keep writing one? If you don't like your story, find out why... go through with a fine tooth comb and be really objective about it (which is the trickiest, since it came from you). If you can't be objective, get someone who's brutally honest to read through for you and they'll be able to tell you what they don't like and it might coincide with something you didn't notice before but don't like either

I hope this has helped! I had other "advice" for you but I've only had one coffee and it's completely flown out of my head!
Good luck!!
Katie