How to add a checkbox to a Content Type and act if box is set to true or yes Module name, 'text_grabber' 
Content Type, 'node' name 'essay'
**When I enable this module, I want a checkbox to be added to the ~/node/add/essay page. So when an user adds a new essay to the site they can select yes on the checkbox that will enable the text_grabber module for that specific essay. 
If the user selects yes, I want to be able to add a js script specific to that essay before it is saved. This will allow me to execute code if event is triggered. Is this possible. 

I think I can add a checkbox using an .install like mentioned here: Using field_create_field and field_create_instance to add a checkbox to a content type

but the other part, what can I do?? In layman's terms, I am not tremendously experienced.
You can add a field in a number of ways:


Edit the content type and add a Boolean field through the UI
In your .install file as you already mentioned
Using hook_form_alter() to add a #checkbox.


If you used option 1 or 2 you can pick up whether the field is checked or not using hook_node_presave() or hook_node_update().

If you used option 3 you'll probably need to add a #submit value to the form which calls a custom function in your module.