At the very top of the board, click on where it says notifications, click on the dropdown to take you to the screen for private messages. On the left you will see a box that says, Send New Message, click that and a new page will open. Put in SortingHat as the recipient. For the subject line, just put request for Sorting Quiz. In the body, you can just put that you would like a Sorting quiz sent to you.
Because you are a new member, please make sure you keep you inbox clear of old messages because there has to be room for the quiz to be sent to you. Once you receive the quiz, you can hit reply and the quiz will be in the reply and you can answer the questions right in the reply. The easiest way to do that is to delete the wrong answers and only leave the right answer to the question. Then just send it on its way back to the Sorting Hat.