The parametric equations for a cycloid are given by

Where 'a' is the radius of the circle.

To plot in Mathematica, maybe let a=1 or some other arbitrary value.

I let a=1 here, and the range can be whatever. I just used 4 Pi to show 2 'rolls'.

ParametricPlot[{u-Sin[u],1-Cos[u]},{u,0,4Pi}]

Does this help?