Help.How to think in oop in this project?

Hello.I am new to this forum.I have just started learning basics of C# and oop and i am a beginner in this field.I am in confusion on how to start visualizing classes,objects etc in oop?For eg:-I want to create a simple "office management" web application but I dont know where should i start from and what should be the classes,methods,fields in this case and where and how should i use inheritence,polymorphism,encapsulation etc. This is a scenario, I want to create following departments in the "office management application": administrator, Web designers, marketing, account.Then I want adminstrator to add employees and their personal details to these departments(in database).whereas employees at other departments could only view their details but could modify only certain fields(username and password). So could anyone suggest me how should I proceed creating web application in this case?What should be my first step?what will be the classes,methods in this case? ...