I recommend you start with reading the Developer Documentation, especially the Hello World Example.
Developing a content feature starts with a UserControl, what you do or what conventions you follow inside that UserControl is entirely up to you. If you want to follow the conventions that I use in development then you should study the source code. There are plenty of examples of how to do almost anything you will typically need to do in development in existing feature code, so the source code itself is the best learning tool and is itself the best developer documentation.
Hope it helps,
Joe