First, in order to get in touch, you are going to need to log in with a google account. You can do that by clicking on the "Login" button in the upper right corner of the screen.
Next, look down at the bottom of the page and what do you see? That is the app launcher for the operating system! When you hover over it with a mouse, the icons get much larger. The most important app to become familiar with is called Terminal. That is the app which really makes Linux on the Web an actual operating system rather than just another web page, like this one.
There are actually a couple of different ways for you to open the Terminal app. First, the Alt+t hotkey will do the trick. (That means you have to hold down the Alt key and the t key at the same time.) Next, you can open any app on the app launcher by pressing Ctrl+Alt+number, where number is the position on the launcher for the app that you want to open. So, the first app on the launcher (which should be Applications) can be opened by holding down the Ctrl and Alt and 1 keys, and the second one (where Terminal should be) can be opened by holding down the Ctrl and Alt and 2 keys.