Stop talking and start writing
There is no silver bullet when it comes to writing (or losing weight, learning to play guitar, etc.). If you want to write, then you need to start writing.
I will cede these wannabe writers one point. Occasionally, when you are just dipping your toe tentatively into the waters, having subject matter helps. For me, the advice to "write what you know" got me going. For others, having a book with writing prompts might be one place to start.
Ultimately, however, if you ever want to write that book that you insist you are going to write "in five years," then you need to get started now, not in five years when you "have the time."
Who has time to write? Ok, there are those few fortunate souls that derive their entire income from writing. Their entire day is devoted to writing, or fighting procrastination to do something else, but write.
Currently I work nine to 10 hour days, in a job that's demanding as hell. It's how I manage to prevent being homeless. I'm also engaged in the most rigorous fitness regimen I've participated in since high school (some 30 years ago). My time to write is at night, if I can manage an hour before the need to crash. Better, I set the alarm to get up 90 minutes earlier than usual, three mornings per week. For me that means getting up at 3:30 a.m. If you visit my other blog, Words Matter, you'll see I maintain a regular posting schedule, rarely going more than 2-3 days between posts. There is usually a longer post up weekly, and then one, or two shorter posts.
For me, writing is rarely a choice--it's just something I have to do. That's usually a sign that you've crossed over and become a writer. Writing isn't talking, writing is setting words down in some semblance of order. Then, it's going back and editing, rewriting, editing, etc., until you have something that exhibits some modicum of effort and care.
That's what it takes to write.
So do me a favor. If you aren't willing to put the time into your craft, don't disrespect my efforts by saying things that you have no intention of following through on. Basically, just shut the fuck up, ok?