Copernicus was not the first to suggest that the solar system was heliocentric; the idea had been tossed around by thinkers for some time. He actually accepted some of Aristotle's thinking which has since been proven inaccurate. He also made a number of errors himself, primarily his belief that the earth moved from east to west in its revolutions. He also believed that the orbits of the planets were perfect spheres, not ellipses.
Most of his thinking came from his belief that the sun, magnificant as it was, could occupy no more appropriate place than the city of the solar system:
In the middle of all this sits the Sun enthroned. How could we place this luminary in any better position in this most beautiful temple from which to illuminate the whole at once?
Copernicus also determined that calculations of the positions and paths of the planets and other heavenly bodies was easier to calculate and more accurate if one assumed that the sun was at the center. He made some errors in his calculations; still his theories were more nearly correct than those which had been extant for hundreds of years.