"I've always wanted to write a book, I just (never have the time/don't know what to write about)". Delete as applicable.
Nope. No, no, no. If you came here looking for a soft pep talk to drum up some enthusiasm and inspiration, then I'm really sorry, but you've come to the wrong place. Worse still, you'll find very little sympathy from me. If you find use in such things, then by all means make your way over to Google Images and look for motivational quotes pasted over a picture of a waterfall.
Because for me, there are no excuses. Think about it: like most of the first world, you probably hate your job, right? If not, then lucky you, but perhaps you are not too fond of the working hours or the early rise it makes you do five or six days in a row.
That's one hell of an inhibiting factor if you think about it. Hundreds of days every year when you simply must be at work at a certain time: terrible if you aren't a morning person. And how many of us make excuses? How many of us simply shrug and say "I want to go to work today, but I just can't bring myself to get up at 6am tomorrow"? It is, of course, out of the question. You simply have to go to work, don't you? Your pay packet and, by extension, livelihood depend on it. In other words, we can overcome our natural apathy and dilly-dallying if and when something truly important relies on it.
So how do you bring that do-or-die attitude to writing? The writing is not yet paying the bills, so how do you approach it as if it does? I don't profess to have any single fail safe method - I don't think anybody does - but I think that's the point, which brings me back neatly to my first point: you can't just wait for inspiration to strike. You can't wait for the muse to descend, for the perfect story to waft over you in a dream one night. Writing is hard, painful graft. Writing is sweat, tears, ground teeth, crumpled notes, eraser residue and fear.
Oh sure, writing is piles of fun as well. Of course there are days where you can pound along through your latest project , feeling like you are weaving gold with your fingers. Even your 9 to 5 has days like that, I'd wager. But make no mistake, those days are cruelly few and far between. Most of the time, there are days where your mind and the paper in front of you will have a standoff over who can stay blank the longest.
But you should never avoid those days, any more than you would avoid a day of paid work because you dread what the day may bring. These days where you feel as wrung out as a tube of toothpaste squeezed dry are as important, if not more so, than the days you weave gold. These are the days you truly build those writing muscles and hone your craft, even if you end up ultimately discarding what you did that day.
Writing is hard, hard work, and there is no magic formula to help you work around that. The end result can be deeply satisfying: there are few feelings as wonderful as finishing a book, and suddenly all your darkest days of doubt and despair turn into bright beacons reflecting your resolve and willpower back at you. It is worth it. But that doesn't make it easy.
Now get to work and write.