how to use selenium for desktop application?

I am working on a retail software where i have to do automation testing of desktop application using selenium. How it can be done using selenium? And we have portal too in which some details are being updated. So the testing will be done for a desktop application and web application. i am fresher i this field and started learning new things. Please help me with this. Thanks in advance. i have completed automation of portal but there are details which are being updated to portal from software.