There are several options available to get started learning how to use WiX.
This help file includes a set of How To Guides that explain how to accomplish common Windows Installer tasks using WiX.
If you prefer to learn by interacting with the community, there is a WiX users mailing list.
If you prefer to learn by using an integrated development environment, there is an overview of WiX editors at http://robmensching.com/blog/posts/2007/11/20/wix-editors.
If you prefer to learn by working backward from a Windows Installer package you have already created, you can run the WiX decompiler (Dark) to convert your package into WiX authoring and then recompile it using the WiX compiler (Candle) and WiX linker (Light).
If you prefer to learn by reading code, WiX is an open source project, and you can look at the source code by reviewing the How to be a WiX Developer topic.
If you prefer to learn by writing code, you can review the WiX issue tracker.
For WiX development assistance, there is a WiX developer mailing list.