KlausST has a good point, besides the torque, you need to know how fast you want to rotate the wheel, as this also determines the required mechanical output power the motor must have.
Just an example:
Steering wheel diameter 0.2 m
required force at 0.2 m radius to make a full turn within 2s: 30N
Torque = 0.2m*30N = 6 Nm.
angular velocity is 2*pi* 1/2 = 3.14 rad/s = 2*pi*revolutions/s
Mechanical power = (angular velocity)*(torque) = 3.14*6 = 19W.
You may need higher power as accelerating from 0 to 3.14 rad/s may require additional torque in case of heavy structures. You also need more power to overcome the losses in the gear box.
The gear box:
If your motor has 2000 RPM at nominal output power. 2000 RPM = 2000/60 = 33.3 rev/s
you require a full turn in 2s, so that is 0.5 rev/s.
Gear box ratio = 33.3/0.5 = 66.6.
If your steering wheel requires static torque, the motor needs to deliver torque without turning. This may overheat the motor as there is no cooling. This can be solved by using a torque limited brake.
Other option is to use a self-locking worm gear/drive. The main disadvantage is its low efficiency (<50%) to enable safety self-locking. you need more motor power to compensate for the mechanical (heat) loss in the worm gear.
There are other solutions when you search for back driving self locking, but these come with a price.