If I remember correctly, around this time of the year, if USYD's already tried to inform you about TSP already (i.e. 99+ ATAR), they'll send you an email where they formally invite you to TSP (last year they sent it to my personal email on the 21st of January). In that email, they to invite you to a TSP meeting during Orientation Week, where they give more information on it, as well as pair you up with your
mentor.
You have to RSVP in this event through an online form provided in the email (here's the
link to it, but it's probably more prudent to wait for the email first).
Note that you can back out of TSP even after attending this event, as I did. (I wasn't very interested in TSP, but I did SSP. You can do both.)