arcahia,
lol. when i first read your post and looked at the picture, i saw the main guy's head and went, 'why would you want to remove his head?
ok, so you want to remove the head in the lower portion of the image and add in her arm there. got it
the answer is clone, plain and simple. start where her arm is showing and simply work from there down to where you can see her hand. it's not that difficult. you can even use her other arm, the one you can see all of and make a cloned image of that and reverse it and add it in over the head. it shld work pretty well though you might have to adjust/move it a bit.
if you go the clone method, start with a brush opacity around 60-80%. it kinda just depends on how things look as you go. overlap your steps as you work and the 60% part becomes 100% with each new clone click. pay attention to shadows and lighting as you work, but dont worry too much about fine detail at first, just get the general shape and outline.
when you've got it fairly well blocked in, you can use the push or smudge tool to straighten up edges. click in one spot then hold down the shift key and click further down the edge and it will auto-paint in a nice, straight line giving you a nice, clean edge.
for adding texture, i often use the airbrush set at a very low density and a very low opacity and dab it on, one click at a time, rather than 'spraying' along a line. also, if you do the airbrushing on a separate blank layer you can do a gausian blur which will make the new 'texture' blend in better.
hope that helps.
craig