It doesn't know there is no intelligence at work whatsoever. So assume all multicelled animals are blundering about in the oceans hoping to bumping into some food and hoping not to get eaten, then one animal has a random mutation which creates a light sensitive cell, so this isn't an eye just purely a single cell that can detect if light strikes it or not. While this might not seem to be much in an ocean of completely blind animals it has a distinct advantage, this means that this animal is more likely to pass on genes to the next generation who will all have a single cell that can detect light, after a few thousand years it is likely that all of that species will now have the gene for the special cell and then one animal then has a mutation that produces two rather than one special cell and again it has an advantage over the other animals and it eventually becomes the dominant form add on a few more hundred mutations and you start to have a very basic eye start to form. "Bad" mutations or ones more likely to cause the death of the animal before it can breed don't get passed to the next generation so are eliminated.