I am not going to write a lot here , but the most important thing, please please stick to the Modern UI.
https://dev.windowsphone.com/en-us/design
My personal policy is to spend ~70% time designing my app for the best possible UX and UI and the rest 30% for development. Developing stuff is easy, designing apps that people want to use regularly is difficult.
Taking inspiration from the top WP apps is the way I went. Evidently I too am just 2 months old into WP dev.