How do you clear everything off a Mac as if it were new?

How to backup everything on my mac through time machine?

• Help! i dont know how to backup everything through time machine.. I tried opening the time machine (in system preferences) and turning it on... then it says to choose the disk you want to use for backup.. there were no disks! i was confused... do i need like an external hard disk to backup every files on my mac?!

• Answer:

  Yes, you want an external hard drive, no point backing up your files to the same drive as that which holds your original data - if it fails both original and backup are lost. Look for something in the 1TB range, they are quite reasonably priced now. You may have to reformat it in Disk Utility (Applications>Utilities folder). Select the external drive, click on the Erase tab and select Mac OS Extended Journalled format. It will ask you if you want to use the disc with Time Machine, click yes. The first time you run Time Machine will take a long time as it needs to copy every file on your hard drive. After that it will be much quicker as it only copies files changed since last backup.