There have been many great answers below covering a wide range of topics necessary for you to succeed at an early stage.

I would emphasize having a strategic plan for how you will survive up to a year and a half without taking a pay check. Once you decided to quit your job you'll have to consider how you will live including housing, food, entertainment (Yes, you need some) ect.

If you're constantly worried about how you'll eat or where you'll be living in a month, it's tough to stay focused on growing your company. By strategically planning and assessing these things you'll free your mind to focus on what's important in your business, not the other things.

And yes, entertainment is necessary. You can't work 17 hours per day 7 days per week. Even Mark Cuban in his book wrote about the bottle of cheap champagne he would drink on Fridays to keep himself sane. Plan to have some entertainment in your life. You can find ways to do it on a shoestring budget.

It would be a good idea to think about your support system. Nobody can do everything themselves both in life and business. Surround yourself with people who you can rely on to be there when you need them.

Other than that, stay lean, stay hungry and get out of the building.