Web development is tricky. That's why web developers use lots of tools to help them with their jobs—and they're always looking for new tools to add to their toolbox.
The browser developer tools which are included in most modern browsers are a big favorite of most developers. Each browser differs in exactly which tools they provide, but they're more similar than they are different.

Inspecting HTML

One great tool is the ability to right click on any element on a webpage and inspect the HTML for that element. Here's a video of how to do that in Chrome:

Inspecting CSS

Another useful tool is the CSS inspector; with this tool, you can check out which styles gets applied to an element and play with them yourself. Here's a video showing how to use this tool in Chrome:
These tools can both help you understand the structure of other websites and help you debug your own websites.
There are lots more tools as well, but those two will give you a great start to being a better web developer. Try inspecting an element on this webpage and see what you find out!