Hmmm... there are so many different ways you can do this! Depending on your skill level it can be time consuming. It sounds like the reason why you can't edit your existing layer is because by default the layer is labeled background layer & it is locked(you should even see a little picture of a lock on the layer). Double click on the layer, you'll get the layer options menu. you can name the layer or just click ok. This will unlock the layer & allow you to edit it. I would advise you to make a duplicate layer copy of your image before you even begin to edit, just in case you mess up.
As for the editing, as I said before, there are different ways to do this. You can always use the eraser & just interchange brush sizes & edge softness/hardness for detailed areas.
Look on the net for some tutorial sites, there's plenty of them!
Good Luck
Narc72