How to Colonize an Asteroid
At first, the distribution of the protoplanets was fairly even throughout the inner solar system. Very small irregularities in the cloud of protoplanets and the action of gravity caused them to begin bunching together - forming the planets we know today, but the extreme mass of Jupiter caused them to stay very stirred up beyond the orbit of Mars and the protoplanets there were not able to join together to form another planet. It is this band of protoplanetary material still there today which we call "The Asteroid Belt".
While the majority of the asteroids are found between the orbits of Mars and Jupiter, many more are found all throughout the solar system. The asteroids with which the first colonists and miners will be interested are those which actually cross the orbit of our earth - or lie just inside or outside that orbit.
This has been a very simplified description of the formation of our solar system, but my intention has been to introduce you to what the asteroids are and how they came to be. For a much more detailed description of what happened, see {REFERENCE INFO NEEDED!}