How do I checkout just one file from a git repo?
It sounds like you're trying to carry over an idea from centralized version control, which git by nature is not - it's distributed. If you want to work with a git repository, you clone it. You then have all of the contents of the work tree, and all of the history (well, at least everything leading up to the tip of the current branch), not just a single file or a snapshot from a single commit.
git clone /path/to/repo
git clone git://url/of/repo
git clone http://url/of/repo