Directed towards centre because motion is uniform, so speed constant, which means no force in direction of motion. Has to be in not out in order to "push" the object round in a circle.
For the magnitude... try drawing a vector triangle showing the change in velocity for a small distance moved around the circle - the 3 sides of the triangle being the initial velocity, the final velocity (the velocity after moving a small distance around), and the change in the velocity. Then you have a velocity change, and remember that acceleration is change in velocity per unit time. And you know know the relation between velocity and angular velocity: v = ωr. That should be enough information to get there...
Hope this helps!