Unless you've been specifically told otherwise, if you've two interviews then wear your best suit. It can only help your cause, especially if others are in casual gear.
As regards the interview, know your CV well. This is all the interviewer will have in front of them. Be prepared for any questions on it and conversations that may lead from it. Don't be afraid to lead a conversation, but don't appear aggressive either. Make sure you know what's going on in the business world at present. Read the business pages of the paper every day for a couple of weeks before it and also a magazine like Business Plus. You'll be able to work that into a conversation in any interview. Finally, know some facts about the company itself, why you want to go there and where you want to be in 5 years.
Good luck with it.