Start with the Elements panel to inspect the HTML and test CSS changes live. The computed styles view shows which rule wins, while the box model diagram makes spacing issues easier to spot.
Follow the evidence
Use the Console for JavaScript errors and the Network panel for failed requests, slow assets, or incorrect responses. Reload with the panel open so you can connect a visible symptom to its source.
When you find the fix, move it back into your source files and refresh the page. Tools are most useful when they help you understand the cause, not just hide the symptom.