To starting hacking into ERPNext, you must have some understanding of how a dynamic web application works. There are hundreds of architectures and frameworks to make web development easier, but at the core there are a few elements that are important to understand.
ERPNext is built on wnframework which is primarily developed for ERPNext but can be extended to make similar database driven applications. wnframework uses Python on the server-side and has a javascript based client for entering data, managing workflow and making reports.