You could start with a circle of radius r centered at the origin that lies in the xy-plane. Then apply a rotation so that it lies in the plane parallel to the given plane. Finally, translate it so it's centered at (a,b,c).
Vela has given you one method. Another is to find two unit vectors u and v that are perpendicular to the plane's normal vector N and perpendicular to each other. Then use
[tex]R(t) = \langle a,b,c\rangle +r \vec u \cos(t) + r\vec v \sin(t)[/tex]