Creating and distributing apps for elementary OS
Major experience with Vala is not assumed, but basic programming knowledge is and experience with similarly syntaxed languages will be valuable. If you're not familiar with Vala, there are great resources—text and video—available for learning Vala on Valadoc.org.
Design is covered in the Human Interface Guidelines (HIG). We reference the HIG throughout this guide and it's important you grasp the concepts proposed there, but this guide is focused primarily on code.