Once you have your Google Classroom account (a Google account on a Google for Education domain) then you can log into Google Classroom and set up your first class. You can set up as many classes as you want but before you do, take a minute to work out how you're going to name them. I've settled on using the Year and Class as the class name (e.g. 2015/16 Year 5) and then I use the subject as the section. So I have 2015/16 Year 5 Maths, 2015/16 Year 5 English etc. I teach more than one class so its easer for me to set it up this way. If you're only teaching one class then you could just set up one class but I think its better to seperate out by subject so that things don't get messy.
When you log into Google Classroom you're shown a list of all the classes that you've created. (As you can see at the top, Google calls these 'Courses'.) If you haven't created a class yet, then this screen will be empty. As you can see from the dialogue below, you can either join a class or create a class by clicking on the + link next to your name.
Choose create a class and you'll see the following dialogue on your screen. Enter your class name and section and you're ready to go.
Fabulous, you've created your first class.
You're now ready to add students to your class.