A fundamental dependency for building Mumble is git, so make sure you have it installed. If you are new to git, make sure to checkout this guide on git's basics in order to be able to follow the given instructions.
The first step in building Mumble is to clone this repository via git clone https://github.com/mumble-voip/mumble.git.
In order to actually build Mumble, you can follow one of the following instruction sets:
Furthermore you might find these helpful as well: