On windows 7+ you can use http://windows.github.com/
Otherwise or alternatively you can use. Tortoise Git http://code.google.com/p/tortoisegit/ Which requires mysysgit http://msysgit.github.io/
if on windows xp older verison you may need to install the windows installer 4.5 Redistributal update. http://www.microsoft.com/en-nz/download/details.aspx?id=8483
If your on 32bit Windows XP get the 32bit versions. Same for node.js and mongodb.
To check out/access git after its installed try right clicking on a folder and doing git clone or git commit or git push depending on what you have done/want to do.