They need to study drilling and sealing underground environments.
That could equal limitless space with almost no leakage and environments protected against solar and background radiation.
First, we need a way for humans to cross space shielded from solar flares, cosmic rays, etc.
Project Orion solved this by using millions of gallons of water as a shield for the crew - 200 odd, in the small ships.
That would appear to be the best way to start out there for sure. Next year they're sending a seismometer to Mars on InSight. I suppose that will allow them to choose a suitable place to dig a home once they have some data.