Sure, I've had several such jobs, the first of which were entry level.
First, they're going to show you how to do things, so don't be nervous about not knowing. They have to introduce you to the office setting, what programs they use, what tasks they need done, etc. You do not have to bring anything (unless they specifically asked you to). Offices have plenty of stationery! If you really want to, feel free to bring your own little notepad so you can take notes on what they tell you -- but they'll supply you with whatever you need/ask for anyway.
Admin assistant office duties usually include filing, answering phones, typing, reviewing or creating documents, maybe some client correspondence/sending out emails, etc (depends what kind of office it is/what they do, but this is the standard list). Filing is easy -- just put files in drawers in alphabetical order. They may give you some material to type up/edit. They may give you a pile of papers and ask you to go through it, highlighting certain sections or typing certain lines of information into a document. They will show you exactly what they want done before you have to do it, and if you have any questions, they should make themselves available.
They'll tell you how to answer the phone, how to put someone on hold so you can get help with a question, and how to transfer the call to somebody else.
They might even ask you to make coffee! I'm not a coffee drinker, so they had to show me how to make it on my first day (embarrassing!).
If you want, ask them what computer software they use if they haven't told you already. Then you can download it on your own computer and figure out a little bit of how it works on your own. Most offices use Microsoft Word, Excel and Power Point, and Outlook for mail, so you could study those.
If you get bored, make little people out of paper clips! I made a whole bunch of those at one office job I had. Good luck!