im new and need help

hi , my name is daniel, Im new here. I havent yet built a bike but im thrilled about the idea. My problem is i would like to motorize my bike, but id like to mount the drive sprocket on the right side of the bike so the motor can use the gears. I havent come up with a good answer for how to gear the motor, AND be able to coast on the peddles when im only using the motor.. Id still like to be able to assist the motor by peddling to get going and stuff but I really dont want to have to be forced to move my legs with the peddles constanly. Has anyone been able to do this? Ive got two bikes to work with, one is an 18 speed with derailur, the other is a 3 speed with shifting in the hub. id use either if it worked .Thank you for your time. DANIEl