What's the minimum amount of software I need to install to get the 'tf.exe' program?
You need to install Team Explorer, it's best to install the version of Team Explorer that matches the version of TFS you are using e.g. if you're using TFS 2010 then install Team Explorer 2010.
2012 version http://www.microsoft.com/en-gb/download/details.aspx?id=30656
2013 version http://www.microsoft.com/en-us/download/details.aspx?id=40776
2019 version https://visualstudio.microsoft.com/downloads/#visual-studio-team-explorer-2019
You also might be interested in the TFS power tools. They add some extra command line features (using tfpt.exe) and also add some extra IDE features.