A lot of people treat choosing their cofounder with even less importance than they put on hiring. Don't do this.
Most startups are not nearly focussed enough. They work hard...maybe, but they don't work hard on the right things.
At YC we have this public phrase, and it's relentlessly resourceful.
Unpopular but right is what you're going for.
So always keep momentum, it's this prime directive for managing a startup.
Many of the best YC companies have had phenomenally small number of employees for their first year, sometimes none besides the founders.