If you already have the MacBook, you don't have to make a backup. Just use Migration Assistant (or Setup Assistant, if it hasn't been set up yet) to transfer the files from your iMac. Read > http://pondini.org/OSX/SetupLion.html
If you want to restore from a backup, first connect the external drive to the iMac and set up Time Machine > http://support.apple.com/kb/HT1427
When the first backup finishes, you will be able to restore the backup onto the MacBook.
To return the Mac to factory settings:
1. Insert the Snow Leopard DVD and hold the C key while your Mac is starting.
2. Choose your language and go to Utilities menu (on the menu bar) > Disk Utility.
3. Select the hard drive at the top of the sidebar, go to Erase tab and erase the drive. If you want, you can choose one of the secure erase options if you want to make sure that your files aren't recoverable,
4. Close Disk Utility and install Mac OS X. When the installation finishes, hold the mouse or trackpad while your Mac is starting to eject the DVD, and turn off the Mac. Remember to give the DVDs to the new owner