The first thing you want to do, obviously, is install git if its not already. Once git is intalled we need to clone the public repository to your local system. At this time of this writing this requires about a 130MB download.
First, position your current directory in the location where you want your working directory to be created. Don't create the working directory as git will refuse to init inside an existing directory. For this example we will clone to a jnode directory in ~/git/.
cd ~/git git clone git@github.com:jnode/jnode.git jnode
Once this has finished you will have a freshly created working directory in ~/git/jnode and the git repository itself will be located in ~/git/jnode/.git For more info see Git Manual Chapter 1: Repositories and Branches
This process has also setup what git refers to as a remote. The default remote after cloning is labeled as origin and it refers to the public repository. In order to keep your repository up to date with origin, you will have to fetch them. See Updating with git-fetch for more info.
When fetch pulls in new objects, you may want to update any branches you have locally that are tracking branches on origin. This will almost always be true of the master branch, as it is highly recommended that you keep your master branch 'clean' and in sync with origin/master. It's not necessary, but it may make life easier until you understand git more fully. To update your master branch to that of origin/master simply
git rebase origin master
Then if you wish to rebase your local topic branches you can
git rebase master [branch]
The reason we're using git rebase instead of git merge is because we do not generally want merge commits to be created. This is partly to do with the svn repo that commits will eventually be pulled into. svn does not handle git merges properly, as a git merge object has multiple parent commits, and svn has no concept of parents. Where git employs a tree structure for its commits, svn is more like a linked list, and is therefore strictly linear. This is why its also important to fetch and rebase often, as it will make the transition of moving branches over to the svn repo much easier.
To learn more about branches refer to the git manual. It is highly recommended that new users to git read through chapters 1-4, as this explains alot of how git operates, and you will likely want to keep it bookmarked for quick referencing until you get a handle on things.
For those users that find the git command line a bit much, there is also `git gui` that is a very nice tool. It allows you to do alot of the tasks you would do on the command line via gui. There is also an eclipse plugin that is under development called egit, part of the jgit project implementing a pure java git implementation.