Your teacher can schedule tests for you. Once these are made available, tests will show up on the test widget on the right side of your home screen under TESTS.
Here there are no tests scheduled. Once a test is scheduled for you, it will show up under TESTS as an upcoming test. It is still greyed out, since you can only access it once the START time has passed
An hour before the test starts, the waiting room will open. This allows you to click on the test link and wait in the waiting room until you can start the test.
In the waiting room, you will see some of the test settings and possibly instructions from your teacher. There will be a timer that counts down until the start time has passed.
Once it does, the start button will appear and you can take your test.
After you have submitted the test, you can return to the home screen. There, you will find a link called 'Test results' that brings you to your test results. It might be that your teacher chose to not release the test directly. In that case, you'll have to wait until he does before you can review your test.