C# Code

hi guys, I am a Beginer Programmer.
I Like to Learn about C# and i started from a week ago.
I Searched in google to find a froum to get some help about C#.
and i found this website.i hope i can find some friend to get help.
For first Question, i want to make a project and i have some question about it.
i have a Main page ( Form1 ) and i have a ComboBox on it.
i made a ComboBox with Readonly( true) and i put a RichTextBox under this ComboBox on mainpage.
now, in ComboBox i have 2 selecteditem.
i want to know how to write code for combobox that if someone klicked on first selecteditem in combobox then he will see in richtextbox for example ( hello ) , if he klicked on second he will see in richtextbox ( bye )?
Can someone please help me?
Thank you so much