Firstly it is important to point out around 9 out of 10 lumps and bumps are not cancerous, but some are, so getting them checked early by a doctor is a good idea.
Testicular cancer is rare and has an extremely good 5-year survival rate of 97%. Most cases occur in men aged 15-59 years. However, it is still very important to check regularly for testicular lumps and get them seen to early.
How to self-examine:
A good time to check your testicles is after a warm shower or bath as the scrotal skin is relaxed. Ideally, check yourself once a month.
Hold your scrotum in the palms of your hands, and use the fingers and thumb on both hands to examine each testicle individually. You should feel a soft tube at the top and back of the testicle - the epididymis which carries and stores sperm. This may feel slightly tender but don't confuse it with an abnormal lump. It has a firm, smooth tube of the spermatic cord which runs up from the epididymis. The testicle itself should be smooth with no lumps or swellings. If you are wondering if one is normal or not, compare it to the other as it is unusual to develop cancer in both testicles at the same time. Note the size and weight of the testicles. It is very common to have one testicle slightly larger, or which hangs lower but any noticeable increase in size or weight may mean something is wrong.
Some more information is here.
A great video showing how to check yourself is here.