I remember when I first started building websites I had trouble understanding what languages were used for what and how to use them. So I'll try to clarify that up for you:
HTML - the base language of the web. No matter what language you use on the internet, HTML will be a part of your website. HTML is what puts all the different languages together into one page.
CSS - The design language. CSS will allow you to position elements, change element colors, font styles and many other things can be done with CSS. I highly suggest mastering CSS as you can do MANY MANY things with it. Even Animation now with CSS3.
PHP - The server-side scripting language. PHP is personally my favorite language just because you can do SO MUCH with it. PHP is what will handle anything like contact forms, form submissions, logins, registrations, database management and everything like that. This is the language you will use if you need to store info in a database.
I hope this helps!