If you wanted to get a new hard drive for your operating system, it's a bit more complicated.
I assume you're wanting to replace the C drive?
One way of doing it would be to buy the new hard drive and connect it up. From here, you could download a free trial of a drive cloning software. The free trials usually let you try it once, but since you'll only be needing it once, it won't be a problem.
You can use this software to copy everything from your old hard drive to the new one, bit for bit. You can then clear/remove the old one if you need to.