I think that I have an appropriate answer for your question.

 

           The most important reason for rotation of the earth is the big bang theory. According to the big bang theory, about 15 billion years ago a tremendous explosion started the expansion of the universe. At the point of this event all of the matter and energy of space was contained at one point. This occurance was not a conventional explosion but rather an event filling all of space with all of the particles of the embryonic universe rushing away from each other. The big bang actually consisted of an explosion of space within itself unlike an explosion of a bomb where fragments are thrown outward. The galaxies were not at all clumped together, but rather the big bang lay the foundation for the universe.

           Due to the big bang, stars and planets are moving with their respective velocities. So Earth has got its particular velocity to move.

           You know that mass has gravitation. So the sun's gravitation acts on the earth. Combining the velocity of the earth and sun's gravitation, the earth has acquired a tangential velocity.

           Thus due to result of tangential velocity because of big bang as well as the law of coservation of angular momentum (as stated by bhuvan & edison) the earth rotates around itself.  

 

       Rate me if you think I'm worth it.