Please give advice on designing and developing the application

(Sorry for bad English) I am junior C# developer and i need your help, advice and whatever. I have a problem with one task that the boss gave me. Nothing complicated, but... I'm just asking for an idea or anything because all my diagrams that I compiled do not suit me. Solid singleton patterns and so on. I need to develop a payroll application, draw up a class diagram, and implement an algorithm for calculating the salary of each employee at an arbitrary time, as well as calculating the total salary of all employees of the firm as a whole. There are 3 groups of employees - Employee, Manager and Salesman. Each employee can have a supervisor. Every employee, except Employee, can have subordinates. It is also necessary to cover the code with tests and differentiate the rights: each employee will have his / her login / password, having the opportunity to view only his salary, and the salary of his subordinates.