Adding Interface to project

Question

Hello all,i wanna add interfaces to the project that i have,but i have no idea when i should use interfaces for what parts of the project,and how to use it.
Also,i want to delete away the UserInterface( ) class and have like,all classes concerend to the Employee eg.the menu,etc to be in the Employee class only,and the other classes too.
I have uploaded the source code already.