1.Each of your professors will post somewhere (the old-school ones on their door, the tech savvy ones on the law school website/blackboard server) the reading assignments for the first week. Read them. It won't be painful.
2. For your first class (not orientation or the silly icebreakers), look around. You're stuck with those people for the next year. Love them, hate them- you'll do both and every emotion in between. They're your friends, rivals, cause of your suffering and support group from now until graduation (and beyond).
3. Get in a comfortable pace. This isn't a sprint. You're not going to impress anyone on what you do the first day, unless you want to be 'that guy/girl' that did (fill in the blank).