I was reading two sites before I posted the question here. One site says something like that the collisions in Universe eventually set the planets rotating, another site says that our planet is formed from some collapsing Nebula or whatsoever and Nebula was rotating when it was about to die and that set the Earth rotating at first place.
The question should be probably better specified. Is Earth rotating because of some continuous force, or because at first place there was some one-time force which set it rotating and the effect lasts until now?
As for the first case, which force is it? As for the second case, that should break our physical laws, because there would be one force, applied a very long time ago which is still in effect and not about to slow down and with a very very long lasting effect. Because thousands or millions of years for rotation is a very long time, isn't it. Or is it slowing down?
BTW: the infinite movement around orbit is bit mysterious too, because you can say that's because of gravity, but gravity itself attracts the things only to one direction, but orbit movement is like parallel with the gravity emitter, and again it is perpetual.