1. Wear a suit. It's still an interview. This shouldn't even be a question.
2. You show up in the morning (most likely), you will meet with likely 4-6 attorneys. You will definitely meet with a partner or two, likely some senior and midlevel associates, and maybe juniors. You may get a list of people who you will meet with - take the time to look them up on the website to find out a little about them - what practice group, any thing you have in common, etc.
3. You'll be asked interview questions. Some attorneys, who may be more old school, will ask the classic annoying interview questions (strengths, weaknesses, etc.). You will absolutely be asked what type of law you want to practice, and you will be asked what you're looking for in a firm. You may be asked about law review/moot court/mock trial, if you're involved. You may be asked about classes. You may be asked about hobbies or interests.
4. You need to be prepared with questions to ask. Do your homework. And DO NOT ask the same 2-3 questions of each interviewer. The attorneys who you meet with will all compare notes at hiring committee meetings, and asking the same 3 questions will ensure you do not receive an offer.
5. You may/likely will go out to lunch afterwards, usually with either two more junior associates or a partner and an associate. Even if it seems more casual, don't kid yourself - it's still part of the interview. You need to be personable, likeable, etc., but don't let your guard down.