Here's a good article on the science behind orbital habitat design. Here are some of the designs that have been proposed over the decades:
Asteroids: provide a ready source of materials for habitat building. The "bubbleworld" or Cole habitat was proposed by Dandridge Cole in 1964. It's creation involves using solar heating to melt the interior of a iron or nickel asteroid and reshape it into a hollow tube. This can be spun to create gravity and soil and water placed on the inside.
Asteroids could be hollowed out in other ways to create habitable space, of course, but given their irregular shape it would be difficult to figure out how to create spin gravity.
Bernal Sphere: It's shape is optimal for retaining atmosphere and providing radiation shielding. If spun, gravity would be generated in a strip along its equator. Gerald O'Neil calculated that a sphere 1800 m in diameter could house 140,000 people comfortably.