Just be interested in what they do, that makes it feel worthwhile for them. Find out what they do and why they enjoy their jobs plus any advice they have. Eberyone wnats legal experience becayse its so competitive, so you did well.
By polite I mean dont be rude as its a place of work, by discreet I mean remembe they cna be dealing with confidential information and soemthings which affect clients, interested because its where you have asked to be. You might be given a file to read, read it and try to get an idea. they wont always have time for you. be respectful of everyone be they secretary , doorman, office boy etc. .
Youll be fine and it will be very quick, most likely youll sit in a few departments. Yu dont need a suit, but shirt and ie or smart casual wll be fine.
have a look at their website before and see the departments. It will impress them if you know what they do. Obviously closer to the time though.