Without more information it is difficult to answer the question. In particular, I wonder whether your description is accurate. Do you mean that you found a brown lump on your scrotum? I am guessing that this is the case since there is no way to see your testicle underneath the scrotal skin. If this is the case, then it is very unlikely to be testicular cancer. Testicular cancer affects the testicles, not the skin. You might feel a lump through the skin, but you would not see it as a dark spot.
That said, if your lump feels like it is on the testicle and does not move when you move the scrotal skin, then you should see a urologist. If the lump does move when you move the skin, it is not testicular cancer, but it might be something else. If it doesn't go away in a few days, go and see your regular doctor and ask about it.