What you need is.....
1. Backup all your important files ( just in case) to an external drive.
[such as files and folders from 'My Documents', Desktop, 'Downloads' and any other files/folders]
2. a copy of all your programs [ The setup or installation CD/Files]
3. a copy of the Operating System [OS]...i.e. Windows XP/Vista/ 7 and more importantly all the drivers required.
Some laptops come with a recovery disk with all the drivers, make sure you have these before you begin.
Now the hardware.....
Most of the laptop will have a small flap/ a part of the bottom that can be removed by removing the screws. Next to the opening you'll most probably find 'HDD' or a picture of a drive that looks like a cylinder.
Once you open that up, you'll...