Each institution has its own list of what to include. This should be sent to you in a letter or you can often find it on the website of where you want to go. Look back into last years postings on this site and you will find a lot of good advice.
Also you will find a book at the MCA called Portfolio Preparation that offers good practical advice
For the portfolio i did i basically used one of those a2 display folders (pay the $100 for one.. if ur gunna be doing design it's worth it) and put in a bunch of sheets of my recent work.
I got sent an entire booklet for things that I had to put in my portfolio, including a 20 page questionnaire, and roughly 10 drawing exercisies that they requested.
Hmmm.... I honestly don't know what I'm going to chuck in my portfolio. It's for the interview at SCA for 'object art and design'. I have drawings and paintings, but none of them have any relevance to object design, they're just random pictures we did last year at TAFE. And the thingo they've sent out about the interview just stated to 'bring your portfolio'.... /