Keep playing with PS, you'll get the hang of it.
I'd suggest you use Free Transform on the tatto layer...hold down CTRL as you manipulate the 4 corners of the Transform box. Place the corners as if they were thumbtacks and you're tacking it to her shoulder. That should give the correct perspective which will make the tatttoo look like it's more on her skin.
After that, you might lower the transparency just a little and then start experimenting with the various Blend Modes. Most likely one of the Blend Modes will help it really look like an image on her skin and not just a cut 'n paste into the picture.
Keep showing us how it progresses. You also might want take this to our Graphics Design forum.