YOu need to know the lever arm's length (or the radius of the wheel). Basically you need to know the distance between where you want the force applied and the center of the output shaft (of the motor or the gearbox, whichever is last stage in the transmission).
THen you just go:
T = F x L
THat is the MINIMUM torque you need to HOLD the load STILL against gravity. If you know the acceleration required then you can do more. (ie. use F=ma, and then use your calculated F in the first equation.)