The brochure "Don't Panic" from the Gauß-IT-Zentrum der TU Braunschweig (GITZ for short) explains briefly and concisely how you can, for example, access the university's W-LAN (eduroam), retrieve your e-mails from the TU Braunschweig e-mail address, print scripts, register for an exercise via Stud.IP or view your achievements to date in the form of a transcript of records in the QIS portal.
As you not only need your y-number to use the IT services, but also, for example, to register for and participate in courses (e.g. via Stud.IP), please do not forget to activate it promptly!