Before we start, let's go over some basic stuff. There are some trivial and not-so-trivial things that you will need to do to get your responsive site working.
The first foundational thing you should learn is using your browser's inspector to emulate different devices. There are a number of tools available in this toolset. Let's look at Chrome; first: click on the Chrome menu in the top-right corner of the browser window:
Next, select More Tools | Developer Tools. Then you can right-click on any element and select Inspect Element: