Follow the instructions on Installing Chromium's depot_tools to download depot_tools (which includes gclient, git-cl, and Ninja).
git clone 'https://chromium.googlesource.com/chromium/tools/depot_tools.git' export PATH="${PWD}/depot_tools:${PATH}"
depot_tools will also install Git on your system, if it wasn't installed already.
git clone https://skia.googlesource.com/skia.git cd skia
Try out more things from the desktop, Android, and iOS-specific Skia guides.
At this point, you have everything you need to build and use Skia! If you want to make changes, and possibly contribute them back to the Skia project, read How To Submit a Patch.