Please have a look at the tutorials. I don’t see much sense in telling you what to click when you don’t know what you’re doing?
Why do you have to use polarimetry for change detecion at all? This is far from being trivial.
Here are some great documents / tutorials if you are new to SAR remote sensing:
Introduction to SAR remote sensing
A tutorial on synthetic aperture radar
Forest Monitoring with multi-temporal SAR data
Agricultural Applications with SAR Data Guidelines for SAR interferometry
Helpful topics in this forum:
Supervised classifications
NDVI correlation
polarimetric approaches