USUALLY, where do cancerous lumps on testes form? Middle, bottom, top? Inside testicle?

Anyone of them. Growth from a tumor can affect any part of the testicle. So you should have a general feel of the size of your testicles and go around all sides looking for either a localized bump(tumor) of diffuse/general enlargement of over 1.5-2 inches which is progressively getting bigger. Then you should get concerned and check it out with your doctor.