Did you review the iFixIt guide on what it will take to replace your current hard drive [invalid guide link]? You will need tools and you do need to be careful as you can break or damage things if your not.
Steps 12 ~ 17 shows the hard drive currently in your system, your hard drive may look different.
Maybe you might find an external hard drive a better option if you have not taken a computer apart before. I do recommend using a 1GB or 2GB firewire 800 external drive as a good option here.
If you do decide to give it a go, I do recommend getting an external firewire 800 case to house your old drive when your finished. But before you start put your new drive in this case and use Apples Migration Assistant to copy your OS and all of your files to the new drive and even check it out by setting in the prefs this drive as the startup disk. Restart your system and see if it's the first listed drive on your desktop if it is your all set to swap the drives. After you finish put your old drive in the external case as a backup.