Well, if there were sort of a magic "one size fits all" answer, we'd probably all be billionaires by now, but unfortunately it's not that easy :)
I've worked for startups most of my career, lots of them from the "no money and running out of the back of a buddy's garage" stage, and if I really had to boil everything down into a general piece of advice, it would probably be "Think big and plan for the future, but focus in the short term, and ensure that you have defined your goals and mission and don't let yourself stray from the path until you have the resources and luxury to do so."
Another way of putting that would be "What problem are you solving, and for whom? Is [random idea #123,129,202] helping you achieve that goal? If not, either stick it on the back burner to rethink or laugh about later (trust me, you'll have a lot of both!)."
In my experience MOST of the real issues I've seen with startups in the past have been due to a lack of focus... comes with the territory, you're doing new, exciting things, the sky's the limit, and the possibilities are endless! It's hard to nail it down to just ONE thing to get started with. But as you get further along, letting yourselves be pulled in umpteen different directions because you have so many good ideas will make it much, much harder to execute well on any single one at such an early stage where you're very resource constrained and trying to get your foot in the door. If you're planning on pursuing VC funding, they will pick up on a lack of consistent vision/focus/mission as well (or at least that's always been my experience, haven't had to seriously talk to/raise VC cash for about 4 years now).
Good luck!