If no points have been spent in Ride, a Ride check will be needed to simply ride upon a tamed animal, with a DC of 10.
If a single point has been spent in Ride, no check will be needed to ride upon a tamed animal. It will be an automatic success.
For all other checks, the DC is increased by 10 if the character has not put a point into the skill.
Ride checks will generally be used to overcome different kinds of terrain while on a mount. The checks will be determined in the design of the area and are not laid out here (though may be later).