This repo uses gclient to manage dependencies, so to build things from this repo you have to first download depot_tools and make sure it is in your path:
http://www.chromium.org/developers/how-tos/install-depot-tools
Then, create a directory somewhere for your checkout, cd into it, and run the following commands:
$ fetch mojo # use --target_os=android if you want an Android build.
$ cd src
$ ./build/install-build-deps.sh
This creates a directory called 'src' under your checkout directory, clones the repository and its dependencies, and installs any packages needed to build.
You can then build Mojo by running:
$ gn gen out/Debug
$ ninja -C out/Debug mojo