I normally have the following in my bag:
snacks - lollies or something as lectures can't get tiring and sugar helps
iPod - for the bus
Pens/pencils - and other stationary as needed like ruler, whiteout
calculator (when needed)
textbooks (if you need to take them, often I just get them out of the reserve if I need them at uni though because they can't be heavy)
Laptop - only if I have more than an hours break between classes as I don't normally use it in lecture/tutes but I can get on with assignments if i have it)
Lecture notes/tute stuff - as needed
notepad
purse
mobile
lunch
bottled water
For your first day just print off any notes that you have been given in advance and a notepad and pens/pencils along with anything they tell you you need like lab coat, calculator etc and you will be fine. They expect people to be unprepared the first week anyway and after that you will find what works for you. I know people who come to uni with a pen in their back pocket and sometimes a folded up piece of paper to take notes.