Normally, gears are used for reducing speed and hence increasing torque (often from a prime mover source). In the majority of cases the desired reduction ratio, power transmitted and shaft speed can be established from the Product Design Specification.
You should also consider the space available for the gears which may affect the number of stages in which the reduction is accomplished. Also, if the gears are on skew axes then special types of gear will be required, see the section on Standard Gears.