Andy Weir gives the same advice over and over, and it's blunter than most bestseller pep talks. He built a hit while holding a full-time job, so his tips are about getting words down.

The Direct Answer
Actually write, stop telling people your story, and don't wait for inspiration.
Weir's core rule is that the only way anyone can experience your story is to read it, per his Advice to Writers interview.

Actually Write, and Don't Wait to Feel Like It

Weir's first piece of advice sounds too obvious until you notice how many people ignore it. Plenty of would-be novelists spend years building worlds in their heads and never put down a sentence. Weir's own path started the same way, with unpublished manuscripts and writing done on the side of a full-time job. The difference, he says, is that at some point you stop planning the book and start typing it.

You have to actually write. Like...put words on a page.

Andy Weir, Advice to Writers

He's honest about how it feels. Some days the words flow and some days every one is a fight, and he says that when you reread the draft later you genuinely can't tell which parts were which. Waiting for inspiration, in his view, is how you end up with nothing. The slog is the job.

Stop Telling People Your Story

This is the advice aspiring writers argue with most. Weir says talking through your plot with friends and family feels great, and that's the trap. It hands you the audience you were craving, which quietly kills your drive to finish. He'd rather you set a rule that the only way anyone gets to experience your story is by reading it, then use that itch for readers as fuel. It's worth noting he wasn't chasing a mass market when he did this. He's said he assumed his only readers would be fellow science nerds and never imagined the book would find a mainstream audience.

He's just as direct about publishing. Weir got rejected by agents before The Martian, so he tells new writers that self-publishing has never been more open, and that a good story will find its way around on its own. He also points out that it costs a writer nothing to try, so there's little downside to putting the work out and seeing whether it catches. To see how this mindset became a working routine, read the full Andy Weir writing routine.

What You Can Steal