Asking advice from photoshop whizzes...

[desertanimal]

Hi guys. So I have this photo and I want to darken the background as I've seen some of you doing. It looks really nice. However, when I darken the whole pic enough to make the background dark, I can't see the snakes well enough to restore their color well, especially in the middle where their tails are. I've heard of doing layers, but I was looking for internet help and couldn't get my layers to work. I'm sure I just don't know what I'm doing. If this were your photo and you wanted to make the background really darkened down and in greyscale, how would you do it?