Searching out specific quotations is a task assigned by your teacher to encourage close reading of the text. Therefore, the best way to help you with the assignment is to show what strategies you can use to find appropriate quotations as you read through the text yourself.
Much of Antigone is structured around "hamartia", a term that in Christian Greek of the 1st century AD came to mean sin, but in ancient Greek means "to miss the mark", especially as in the sense of an arrow that irrevocably leaves the poem and speeds in a wrong direction to miss its target.
There are several important decisions in Antigone that can serve as examples of hamartia, the irrevocable choices with bad consequences desired in this assignment.
Each character makes one or more of these choices. Antigone, in her first decision to bury Polynices, and then again in her second decision to attempt to bury him again is an example. Creon's initial decision concerning the burial and his decision to wall up Antigone alive are both examples. In each case, you should search for the line in which the character forcefully states his/her intention to commit the specific act no matter what the cost or opposition.