I just had a huge deja-vu when starting this article: I felt like I'd already talked about 'The One Thing' here on WIP. But a quick search suggested I hadn't. After a minute I realized I'd written a script for a video about 'The One Thing' - back in freaking april. (It's a funny story. I should tell it someday.)
Anyway - 'The One Thing' by Gary Keller with Jay Papasan suggests that you should figure out one thing to do and go do it. It's as simple as that, really. But since they wrote an entire (200-something page) book about it there's bound to be more. And there is - they tell you how to go about it.
First, ask yourself any question in the form of "What's the one thing I can do to..." (get out of debt / increase sales / get healthy etc), and figure out what the answer is. Then start doing that, at least four hours a day.
Second, ask yourself what you want in life generally: "What's the one thing I want to do some day", then figure out "based on that, what's the one thing that moves me towards that goal that I can do in five years", then "this year", then "this month", then "this week", then "today", and then "right now". And go do that.
You can ask these questions regarding all and any aspects of your life (no one expects you to put your health at risk while learning to code for example). Subjecting yourself to this questionnaire will give you a sense of what you want in live and what you can do to attain it.
When it comes to practicalities, the authors suggest you should structure your day so that at least half of your working time goes actively and in a concentrated matter into doing your one thing. You should use so called "time blocking" for your one thing and make sure you never schedule any appointments or other tasks to that block. (I talked earlier on a video about getting things done about two-hour blocks to do your tasks; Keller and Papasan suggest you do your one thing a four-hour block every day.) Learn to say "no" to all people and activities that threaten to jeopardize your precious one thing block. And from time to time, arrange larger time blocks (think 24 hours at least) during which you're completely isolated, have food and water to keep you going, turn off your phone, and make sure to tell anyone who might err and disturb you that you're not to be disturbed during the block.
And remember: does Stephen King get to live the live he lives (writes fours hours every morning, then spends the rest of the day as he pleases) because he is Stephen King, or did he become Stephen King because he lives the way he does?