while it is of course not necessary to wear a suit and tie or a fancy cocktail dress, if the occasion does not call for it, it does not hurt to be careful when choosing an outfit.

women might wear a sharp summer dress, a bit of jewelry, and nice sandals and be perfectly dressed for a first meeting - dramatic make-up and heels are not needed, but a nice hairstyle and perfume always go together well.  

as far as men are concerned, what normally works is finding a comfortable style that reflects care and a bit of forethought - this might include a nice button-down shirt, and jeans, or a simple well-fitting t-shirt, jeans, and clean shoes. these choices will go a long way to give a good first impression.