Generally we recommend you use one of the latest versions of GIT.
No GIT client yet
If you have no GIT client yet, get the latest version of the official installer.
I already have GIT
If you already have a GIT client installed, please check the page Supported platforms (for bitbucket) from Atlassian, and make sure that your versions is has been tested against our Bitbucket version. (To grab the current version of the Magnolia Bitbucket instance, look at the bottom of your profile page.)
If you have an too old version, please upgrade.
OSX: Use homebrew, macports, or simply the official installer.
On Windows: check this out:http://blog.tfnico.com/2012/04/my-git-setup-on-windows.html
On Linux: you're a big boy, aren't you? (seriously though, links/instructions welcome. Please update this page)