It depends what type of agency they are, but you're probably safe with smart casual and something that shows off whatever aspects of your look the agency would be interested i.e. probably not jeans and a t-shirt. If they're a fashion agency, then maybe something that shows the type of fashion you like, but don't go overboard.
No itd glamour/ promo ?? Soo casual shows of figure?
Wearing something comfortable and smartish casual. Summer skirt/leggings and top that shows your curves, but don't go overboard. I believe if it's for a glamour agency that does topless work then they may ask to see you topless, so probably a separate top and skirt not a one piece.
Wear something that shows your good features but that you feel comfortable in.if it's for dresses wear a nice dress,if the casting is for a model jeans then go in some nice fitted jeans.of course if it's a lingerie casting don't just rock up in heels and lingerie...you might get a few funny looks on the train