The main difficulty students experience in writing an outline of the main events in a book is also the central reason why teachers assign such projects as homework. The point of the assignment is developing a skill which will be useful in every aspect of school and business, the ability to summarize succinctly and to distinguish important or crucial events from less important ones. Thus you main task is thinking through what constitutes an important event and what makes it important.
The first way to decide what to include in your outline is to think about characters. The protagonist of To Kill a Mockingbird is Scout Finch. The significant events of the novel are those that that have a major impact on her life or intellectual or emotional development.
Next, your outline should concentrate on the two main plot lines, the one concerning Boo Radley and the one concerning Thomas Robinson. What your outline should do is cover in chronological order the main events in these two narrative sequences.
In the Boo Radley plot, particularly important events are the ones in which the children slowly become aware of Boo's acts of kindness and come to learn that rather then being a monster, he is more unfortunate than monstrous.
In the main plot, revolving around their father's defense of Thomas Robinson, the essential elements of your outline should cover the way Scout and Jem evolve from children merely following in their father's footsteps to people who slowly start to understand and stand up for justice themselves.