Talking to the guys yesterday, compsci seems to be useful (CSCI 114) and recommended. Don't know about easy unless you're good with computers though, i was told it would be used a lot later in the course
It took me a week to get mine after submitting the form, but that was through high school and didnt require official documents to prove who i was. It could take you longer i guess, and you'll have to provide birth certificates (im not sure how you would supply them, i imagine you'll have to mail...
Tax file number is the number you give to employers so they can give the Taxation Department a record of your earnings, how much you pay in tax and whatever else. I assume HECS needs it so they can keep track of how much you earn after graduation. To get a taxfile number theres instructions on...