In fact, git-tfs works only by cloning a TFS repository.
I just hope that you've not a lot of branches to checkin into TFS otherwise it will be long an boring but doable!
First, create an empty repository in tfs.
old solution (still valid if you want to take this way...): and clone it with tfs. You should be able to do it that easily and should have only one commit with no files in this new git repository.
Now, add a remote toward your existing git repository and rebase the remote master onto the initial commit of the new repository.
new solution: Use this script
Then use 'git tfs rcheckin' to commit all the history. It should be very long :(
After that, if you have branches that you want to commit, and if you have the last version of git-tfs 0.17.1, extract the good revision and create a tfs branch with the command :
git tfs branch Name_of_my_branch
and rebase all the commits of the git branch and use rcheckin with : git tfs rcheckin -i Name_of_my_branch
Do that for all your branches and that should be good...
But I should add that if you've got merge commit, they will all be lost in the process :( You won't have a as beautiful repository than you're existing git repository
PS :
I'm really sorry for you to be obliged to use TFS after having used git
Can't you use the new feature of TFS 2012 that will be released soon and which permit to have a git repository in TFS? http://blogs.msdn.com/b/bharry/archive/2013/01/30/git-init-vs.aspx and http://www.hanselman.com/blog/GitSupportForVisualStudioGitTFSAndVSPutIntoContext.aspx