The way to determine what scenes are significant is by considering what the scene reveals. Things to consider are whether or not they reveal/develop characterization, reveal a theme, or plot development. Since formatting on eNotes limits space, below are a couple of scenes and a discussion of why they are important to help get you started.
I would say that the very first scene of Antigone is one of the most important. It is particularly important because it helps develop both Antigone's and Ismene's characterization. We are able to very quickly see that Antigone is characterized as a very driven person. She is very quick to judge what is moral and immoral. She is also extremely strong willed, and we may even be able to characterize her as impulsive. These character traits are portrayed through her decision to fight against injustice, particularly unjust law, by burying her brother, as well as through things she says in this scene. In addition, her decision also relays several central themes in the play, including injustice and impetuousness. In contrast, Ismene is characterized as very frightened and very submissive. It is evident that the things she has gone through have shaped her character to the point where she fears death and more loss of family members. Hence, we see that the first scene is a very vital scene.
Scene IV in which we first meet Haemon can also be considered a very important scene. One reason is that this scene particularly reveals Creon's characterization. The scene reveals him to be an arrogant tyrant, one who believes in absolute rule over his citizens, as we see in his lines, "Should I rule this land for myself or for others? ... Isn't the city thought to be her rulers?" (747-749). In addition, Creon's nature in conjunction with Haemon's view that he is acting wrongly helps portray another central theme concerning the dangers of tyranny.