I think although writing it is often a passion and may not seem like work it is still a job and as such then it is good to have some discipline;-
Decide what it is.. fact or fiction
Pick something you have passion about
Sketch out a loose framework
Set yourself a target, every day, every second day or every week - something that is achievable, keep the old writing muscle exercised.
Start writing it ASAP...
Consider a short story as a way to start
NEVER ever let anyone discourage you since writing is to a large extent a solitary pursuit...
There is the creative side of it, the writing writing and then there is the business side of actually trying to sell it.
Figure out what will make it different from the competition
Consider perhaps blogging the writing of it
Build up your Social to engage your readers.. Hubpages, Facebook, Twitter
Consider writing a taster chapter and seeking feedback from a few publishers, or magazines
Look at Print on Demand
Look at the move to EBook
Remember the 3G's
Get it written, Get it out there, Get it making money
Talk to me direct if you want more then this since I am 80% towards setting up my own publishing house here in Ireland and am happy to provide pointers...
Good Luck :-)