I know how a earth revolves in it's orbit. it is because of centripetal and centrifugal forces. so how does earth rotate in its axis?

Nearly 4.5 billion years ago, our solar system formed out of a spinning gas and dust cloud. The cloud started collapsing due to immense gravity. As it collapsed, it began to spin. This is due to conservation of angular momentum.

 

Thus, when Sun and all the planets formed, they began rotating around their axis. This was the inherited characteristic from the spinning cloud out of which the solar system formed.

 

Now, there is no unbalanced force acting on planets, like friction, etc. Thus, the planets continue to spin due to their inertia.