why and how planets rotate?

Answer 1

Conservation of angular momentum. The planets formed from a giant collapsing cloud of dust & debris in space, and it's natural for something that forms in this way to start rotating -- as it shrinks under its own gravity, the rotation speeds up like a spinning ice skater pulling in her arms. Since there's practically no friction in space to slow them down, the planets are still rotating even though they formed billions of years ago. (An object in motion will tend to stay in motion unless some force acts on it to stop it.)

Answer 2

One consideration may be the core of the planet. A liquid core may help a planet rotate about it's axis like a motor.

Here is an interesting look at how the planets rotate about the sun and axis based on conservation of momentum. This planet calculator as shows how oblateness is related:
http://www.a2dvoices.com/realitycheck/Coriolis/

Answer 3

Most of the planets rotate on their axes from west to east nearly in the same plane with slight deviations.They also orbit the sun from west to east almost in the same plane and direction as the sun rotates.
Planetary orbits lie at regular intervals from the sun, i.e.,roughly twice the distance from the sun as its next inward neighbour.
The rotation of the planets around their axes relates to the origin of the solar system itself.It has been generally accepted that the sun and the planets have formed from a revolving disk of hot gases known as solar nebula.
Due to absence of any friction in the space to slow down the spinning, they continue to rotate.