The three sections at the top are controlled with sections on the template:

I want to Serve text is controlled using the content area of the template:

I want to serve buttons are controlled by the EVENTS module. You will add the event, and then add the Missions parent category AND the "nationally" or "internationally" sub category so that it shows up in the correct button

Event detail page is automatically populated from the Event module with the Event info. The "Sign Up" button is populated by whatever link you add to the "register" field