new to threading

can anyone show me an example of how to start one task when another task finished? i tried looking for examples, but my program is simple and I dont need anything complex, mainly because I dont understand what theyre doing. for example, lets say i have a form with a button and a textbox. when i press a button a messagebox appears. then text appears in a textbox of the form, but ONLY after i press ok on the message box? I know how to update text in a textbox and get a messagebox when i press a button, but i want to see how i can get the text to appear only when another task is finished because Im trying to figure out threading somehow... thanks.

This might help.