I agree with what @Shaun_OfTheDead; has said
Good preparation is the key. If you're going to show your work, you need to know the strong points of every design/work that you've done (and their weaknesses too, of course, and how to turn those weaknesses into strong points too )
If you're presenting yourself to a company, it is also good to have a general feeling of their principles and goals and if you can have some info of the people you're meeting too.
Always look clean and professional but don't overdress. If the company is very formal, you'll need a proper suit but if the company is informal, casual clothing may be enough and they may feel a bit overwhelmed if you wore a full suit. In any case, don't wear jeans