Tough for an intermediate, and extremely hard for a beginner, Photoshop is an elephant of a program. You need to use the Lasso Tool, and carefully trace around the head, body, whatever it is you want to copy....if you hit control + you will have better luck, as you will be able to focus in better on the pixels. Copy that (control C)
Create a new file (control N), and don't resize what it gives you, as it will be the perf size of the head, body, whatever you cropped. Paste (control v) the image on the new blank sheet.
open the file you want to paste on to, and copy the new file you just created, and paste it onto what you want to paste on to....the tricky part here is the sizing....you are best to resize the "new" file you made....but it's important to do that only after you have seen it pasted, and not before you paste it from the first original picture.
that's the short of it. i have 2 books on it, and buy the photoshop magazine now and again, and i haven't even come close to looking even half professional....
still, hope that helps.