The information will likely be dependent on what motor you are looking to couple and what size shaft you are looking to couple it to.
One general thing to keep in mind with using a shaft coupler on a motor shaft is that many motors are not designed to withstand substantial side load so you must be careful with how you support the shaft if you are using a non-flexible coupling and your application may side load the motor.
As RC mentioned, many teams have used shaft couplers before. For some of the kit motors it may be helpful to adapt them to a standard size if you can't find the coupler you need to connect your motor to your desired shaft. Andymark makes adapters to turn both a
CIM shaft and
Globe shaft into a standard 1/2 keyed shaft.