It's not much. Once you arrive, you go to the immigration bureau (Aliens' office or something like that) on Burgh Quay. Bring: passport, letter of acceptance, letter confirming that you are/will be an entering student (get from student record's office or office of the registrar/similar office from your college), bank statement (requiring you to open a bank account), receipt showing you've paid tuition and that's all I can remember. Don't forget something because it's a pain in the ass to have to go back. The queue can be pretty long.
You don't have to do it right away. Usually if you get here in September or something, you have til November/December to do it. And even then if you're a little late, it doesn't really matter. So you can take time to get settled first so you can go in with all the necessary documentation.