It’s the right hand rule. Curl your fingers in the direction of rotation and your thumb points in the direction of the spin axis.
For the Earth, the spin axis is up, the positive direction. This is the same as the net angular momentum of the solar system. When the solar system condensed out of the nebula we started from, there was a net positive angular momentum with a spin axis up.
Our spin axis is titled at about 23 degrees from the orbital axis. This suggests maybe we got tilted in an early collision, like when the moon was created by a collison with some other large body.