Take a color photo and convert it to grayscale and then erase the area that I want to remain colored?

I am trying to use Gimp to do this but for some reason, it is not working. I try to erase the area I want to remain colored and it turns white. I must be missing a step or something. This is new to me, but I have seen it done before. If someone could list the correct steps that I must take, it would be appreciated. Thanks.