There are three main theories: that they used levers to hoist the blocks up the side of the pyramid, that they used a single long ramp to reach the top, or that they used a ramp that circled around the pyramid. They were built during the time the Nile flooded by individuals who were usually selected by a means similar to the draft. They were slaves. The pharoah was considered a god, and recent excavations have found burial sights near the pyramids of workers (inscriptions on the walls often brag about the work they've done). No slave would have been buried so close to the pharoah. Remains of fish and other foods were found that would not have been "wasted" on slaves. They did not have wheels, but they may have slid the blocks on rollers, with men pulling the blocks with ropes.
To level the base of the pyramid, it's believed that they simply cut channels in the ground, then filled those with water. The water would have been level, so all they had to do was level the ground around it to get a flat surface. They had triangular plumb bobs, which they could place on the stones, and if the plumb bob was correct, the stone was flat. They also had plumb bobs which were like an upside-down L. They could attach the plumb bob to the short end of the L, place the long end against the stone and determine if it was level up-and-down. Finally, they would take three wooden dowels of equal length. They would attach string to two of them, set them on the stone, and run the third under the string. If it exactly touched the string, it was flat. If not, they could tell if there were low or high spots on the stone where they'd need to smooth it.