Use CarbonCopyCloner or SuperDuper! to clone your internal HD to an external.
Then follow these steps:
1.Boot from install disc (press power button, immediately insert install disc and hold down c key).
2.Select your language.
3.Open Disk Utility from Utilities menu.
4.Select the HD in the left pane (the manufacturer’s name and the HD model number).
5.Click on the Erase tab.
6.Choose the Format: Mac OS Extended (Journaled).
NOTE:if you want to zero the HD (which I always think is a good idea, but does take time), click on Security Options, click the button for Zero Out Data, click OK.The larger the HD, the longer this will take, in some cases several hours.
7.Click on Erase and wait for the entire process to finish.
8.Click the Partition tab.
9.Click on Volume Scheme and choose the number of partitions (normally just one unless you have other plans).
10.Click on the Options button (located underneath the Volume Scheme pane) and select GUID.
12.Quit Disk Utility and continue with the installation process to install OS X.
13.Restart, connect the external HD, and run Setup Assistant to restore your apps.