Answer 1

HTML is a markup language that is used to ORGANIZE a web document through a series of tags that your browser interprets.

CSS (cascading style sheets) is what we use in order to style our documents as we see fit. You apply its rules to the various elements on your website, and it'll present your data in the style you want.

As for how you use them, you'll want to start by learning them. Once you learn them, that question will answer itself.

https://www.sololearn.com/Course/HTML/
https://www.sololearn.com/Course/CSS/