This is very important for young men, as testicular cancer is one of the leading causes of cancer-related death in males under 40. It is also a good cancer to detect early, as the treatment is so successful if started early.
The key message is do not be embarrassed - if you find any find a lump make an appointment with the Doctor.
Roughly speaking you should self-exam in a relaxed environment, once a month. The link below takes you to a printable leaflet and explains in detail. http://tcrc.acor.org/tcexam.html
If ever in doubt consult a doctor.