The solar system formed when enough of the material drifting around in space started to clump together due to gravity. As this happened, gravity and collisions caused the relative motions of most of this material to tend to cancel out, leading to the clump having an overall velocity and a spin (that’s just a result of the physics of the situation).

As the clump continued to contract, most of it became the sun, while other bits continued to orbit this, contracting independently. Hence the planets and other bodies in the solar system generally sharing a common plane and direction of orbit.

Other events such as collisions and large bodies passing by could change this.