how to get the base class to access the appropriate derived class

[vtk]

I need help in being pointed in the right direction
I am trying to get a base class to choose the
Appropriate derived class so it can preform the
Correct mathematics. For example you enter in an
Employee type (commission or hourly) based on the
Type ot chooses the correct derived class, the base
Class would jist hold general information
I understand the base class but what i dont understand
Is how after entering employee type to the base class
It can access the derived class
Hope this makes sense and i didnt supply my code
Only because im looking to understand the concept
Thank you ahead of time for any help