Your first task is simply to ensure the existence of the organization. And that means keeping the money in the account.
If you're in the investment phase and attracting investor resources, your job is to show a cool performance and make sure investors appreciate that and give you the next rounds of investment.
If you're making money, make sure you're clearly in control of your P&L.
The second task is to get the best team on board and invest in their development and motivation. As Jim Collins said, "It doesn't matter where your ship is sailing if you have irritation on your ship or just the wrong professionals."
And, of course, the third challenge is to define a strategy and execute it.