Assuming you have git installed, you first have to get a copy of the Git repository:

git clone --recursive https://github.com/LMMS/lmms.git

This will clone the master branch, which can be unstable and have bugs.

If you wish to switch to the current stable branch (for the purpose of compiling LMMS by yourself):

cd lmms
git checkout stable-1.2

For instructions on how to compile LMMS, visit Compiling.

If you want to update your copy simply type

git remote update
git pull
git submodule update --init --recursive

inside lmms-directory. The first step usually can be omitted.

For browsing the Git repository easily you can also use the Git Web interface.