How do you find the conjugacy classes of rotations and dihedral groups... I dont even know where to begin...

for eg.

Conjugacy classes of:
$\displaystyle
R8 ={1}, {r}, {r^2}, {r^3}, {r^4}, {r^5}, {r^6}, {r^7}$

and
$\displaystyle
D4= {1}, {r^2}, {r; r^3}, {fr; rf}, {f; fr^2}.$

please explain in detail and from the start... with the theory part ...

Thank you in advance...