Find out the basics of text editors
Text editors come in all many different forms, but at their hearts they all do the same thing - allow you to edit files that are composed of straight forward, everyday text. HTML & CSS files are like this, their contents are simply plain text.
Your computer may already have a text editor on it. If you're using a Windows based computer then you'll likely have Notepad (Programs > Accessories > Notepad). If you're using an Apple computer then you'll have VIM already installed (Applications > Utilities > Terminal and then type in VIM and press return). Finally, if you're running Linux (bonus points if you are) then you'll certainly have one of either VI/VIM, emacs, Kate or gedit.
Not all text editors are created equally and whilst VIM is a truly splendid text editor, it has a steep learning curve. Unless you can already use it, then we'd recommend something else for now. You can always come back to VIM later on.
If you're running windows then Notepad has the opposite problem to VIM - it's too simple. You'll be fighting with it all the way to use it to do serious webcraft work.