The University will contact you by email or post to let you know when and how you can collect your IT account details. This usually happens shortly after your application has been processed and you have been accepted.
This email or letter contains your Personal Record Identifier (PRID) which you'll need to set up your account.
Once you have your PRID you can then set up your IT account.
If you haven't got this email and your start date is soon, contact your line manager.