what is rudys first victory and why is this win so important for him?

I'm not sure what chapter you are referring to. In The Whistler and the Shoes, Rudy leaves a Hitler Youth meeting covered in manure, blaming Franz Deutscher. Rudy and Liesel agree to steal something as a way of giving Rudy a victory. Liesel brings Rudy to the mayor's house with the intention of stealing The Whistler, though Rudy things they are trying to get food. Liesel climbs through an open window and gets the book; they race off at first but Rudy loses her shoes and has to go back and retrieve them. When they get to their homes, Rudy for the first time calls Liesel "book thief."