transferring hard drive to another macbook

Ok well long story short I had water spilled over my macbook which I bought in the summer of 2008. My mom also has a macbook which was bought in December of 2009. she gave me her macbook while she purchased a new one. I am trying to figure out how to transfer my hardrive (which was not damaged) to my "new" macbook. I really don't understand macbooks that well so if someone could please explain to me step by step how to transfer my hard drive to my new computer I will be so grateful. ( I also took my computer to the apple store and they tried to literally switch my hardrive out from the old and into the new but they could not get it to work.) Thanks 🙂