Brand the app with your logo and colour scheme
Decide on a pricing scheme for your app
Digitise your wireframe and start building your prototype
Plan your pitch video
Work on your prototype based on your wireframe (just google Appinventor and your task or check out these tutorials)
Review popular apps on the app store, their colour schemes, logos and navigation design to see what you like and what you don't like.
Review previous year's app pitch videos to see what works. Study Buddy was our favourite pitch video in 2016