Look, I'm 13 and have upgraded various family members' iMac RAM, a few MacBook Pro hard drives and RAM, my own hard drive etc.
You have NOTHING to worry about, plus it is technically still covered by AppleCare. All it is is a few screws off here, take out the hard drive and pop a new one in. Then case back on and screws back in. The iFixit link mentioned in the post before is very helpful, there are many tutorials that are easy to follow there.
Remember to ground yourself though by touching a bit of the metal casing before touching and electronics on the inside. Don't want to short anything out!
Good luck, it's way easier than it looks. You don't want any so called 'geniuses' taking apart your computer do you? 😛