Here's the guide for replacing the drive: [guide|7555]
If you have another Mac, put this machine into Target Mode and see if it can be repaired using Disk Utilities, and transfer your files if they can be accessed till the new drive is installed. If not let us know as the new drive will need to be formatted and a system installed, then using either an external enclosure or a Universal adapter such as this: http://eshop.macsales.com/item/Newer%20Technology/U2NV2SPATA/
transfer your data if it is accessible.