We recommend Counterpath's free (X-Lite) as a good starter softphone. While it's not the only softphone we support (there are others listed here), X-Lite is available for the three main operating systems out there, so it's easy to find the one you need.>
Clicking on those links will download tested and approved versions of Counterpath's X-Lite Softphone.
After you install the softphone and run it, it will require you to fill in some basic fields in order to get things to work properly. Check the new user instructions page for instructions on how to configure X-Lite