Why does the earth spin clockwise?
Earth's Spin

Earth rotating clockwise is the result of a chain reaction that started when Earth's star formed as the result gas clouds collapsing. During the collapse of the gas, one direction was shorter and a disc formed. Due to the law of conservation of angular momentum, the disc gained an overall spin, which was passed to all the objects of notable mass within its solar system; these objects are commonly known as planets. As a result, all planets within a given solar system have the same spin as the star in it to start. That said, the axis of a planet may do a 180-degree flip on its axis at some point, and if this happens, like Venus, it would spin counter to its native spin; in fact, in the case of Earth, it appears given there appears to be evidence that it's has flipped in the past, that the Earth's axis has already flipped at least twice, since it's current back in sync with the Sun's spin.

Might be worth noting that all rotating bodies that rotate clockwise when viewed from the southern hemisphere rotate and counter-clockwise when viewed from the northern hemisphere. Also, since the Sun is not solid on the surface, it does not rotate as a solid body; meaning it rotates faster at its equator and slower at its poles.

Earth's Axis

As for the reasoning behind, the axis of the rotation, as pointed out in other answers, this is the result of Earth's past impacts with objects of enough mass to produce the shift from a zero offset from the stars axis, to the current axis of the rotation. Future impacts would be able to change the rotation.

Will the Earth's axis flip?


  Scientists have found evidence that the Earth might have flipped over
  in the past, completely shifting the orientation of its poles. The
  theory has been around for years; that a large mountain range or
  supervolcano might unbalance the spinning Earth. Over the course of
  millions of years, the Earth would change the orientation of its axis
  until the object was balanced at the equator again.


Current Axis of the Planets in Earth's solar system