Choosing an appropriate motor

Hey. My project involves a motor which is driving a load of 20 kg through a pair of gears. A small gear of probably 100 mm diameter on the motor drives a big gear of 300 mm diameter. The load will be over the big gear. The motor provides azimuth movement to the load. I am a bit confused about the sizing of motor specifically the "torque"of motor. I have visited may pages about the torque calculation but they all calculate the torque with gravitational force keeping in the mind. In my case motor is not supposed to work against gravity instead it is supposed to provide enough torque to overcome the inertia and friction force. How to calculate the torque of motor in this case?
Please help . Thanks