Determining selectors for customizing css

I seem to consistently have a problem determining the correct selections to override the CSS in style.css

I need to eliminate or reduce the 84 pixel top and bottom padding around the masthead as illustrated in the screenshot.

I have three questions:grinning:

1 What is the CSS needed for modifying this.

2. What tools can I use to determine the selectors

3. Are you aware of any tutorials focused specifically on determining the selectors needed to override CSS?