we need to choose business roles based on the activities that one needs to do. observe the standard roles like sales professional and discuss that, with that standard role , are all sales executives can perform their assigned tasks. if not then we need to alter the roles first by copying it. once you login with the role, you will come to know what can we do and what needs to be added/changed.
in order to adjust the role, one must have enough idea on navigation bar profiles, work centers, logical links ....etc. so that you can deactivate what you dont need and can create/add what you need.
regarding the role configuration, i think shobit has given enough explanation.