Typically in residential settings you distribute power to the sub-panel by installing a double pole circuit breaker in the service equipment (main panel) and running a 4 wire feeder sized according to the calculated load to the sub-panel from that breaker.
Wire size is determined in several ways depending on calculated load and the distance from the main panel to the sub panel. The two most common ways are to pick a wire size that will supply the calculated load. CAUTION...In order to do this you need to have a thorough understanding of wire types and the ampacity they can be used at and applications they may be used in ..ie..wet location,direct burial etc..
Once you do that you install an overcurrent device (circuit breaker) in the main panel or whatever panel you are taking power from that will protect that wire size you chose. The breaker should not exceed the rating of the sub-panel.
The second way is to look at the panel rating. If that rating exceeds your calculated load (it should) you can pick a wire size that will meet the panel rating in amps. For example if my panel is a 100 amp panel then I would pick a wire size that will supply 100 amps or more. I would then protect it with a breaker that does not exceed 100 amps because the sub panel rating limits me to 100 amps.
Note the wire size ampacity can exceed the panel rating but the protective breaker should not.
Now this is a very basic summary there are many exceptions.
