Replacing a LapTop Hard drive

I need to get a new hard drive for this laptop, i don't know what to look for or how to replace it so i was wondering if someone here might be able to help me out. I will be buying it off of NewEgg, and i would prefer a cheaper one as we do not have a lot of money. It's a windows vista laptop from 2007. i can get the computer specs if you tell me how, but i have no idea how to find them. software is my specialty, not hardware lol.

If anyone can please help me i would be very grateful, i want to give this Laptop to my little sister on her birthday but it needs a new hard drive  .