The cerebral arteries arise from an arrangement of blood vessels known as the Circle of Willis.
Find the vertebral arteries and the internal carotid arteries inferior to the skull and follow them into the skull. Note how the 2 vertebral arteries become the single basilar artery. This is lying on the clivus of the occipital bone ("clivus" is Latin for "slope").
The orange arteries coming from the basilar artery will supply blood to the cerebellum. The basilar artery ends as a pair of white posterior cerebral arteries.
The internal carotid arteries wind their way through the bone to pop up posterior to the orbits. They continue as the purple middle cerebral arteries and the blue anterior cerebral arteries.
Notice how on either side the white posterior cerebral arteries and the middle cerebral arteries are linked by green posterior communicating arteries. If you look very closely you'll also find a short green anterior communicating artery between the anterior cerebral arteries.
That circle that you now see; that's the Circle of Willis.