Before dual booting a macbook, the drive will need to be split into at least 2 partitions. This tech-recipes describes the sequence for doing this.
In deciding to dual boot my MacBook, I knew that I would need at least two partitions. Creating a new partition on your one and only hard drive is not an easy nor quick thing to do. The OS lives there so you can not partition the drive from within OS X itself.
In splitting your hard drive into partitions, you will lose all your data on the hard drive. Yes, all your existing data will be destroyed. So backup your info.
If you do not want to lose your existing data, you can add another hard drive somehow. A firewire or USB external drive is easy; however, it is very cumbersome on a laptop system.
Deciding how to backup your existing system is the tough thing. Creating the partitions is pretty easy.
1. Put in the OS X DVD and reboot
2. Hold the C key during the boot process. This will tell OS X to boot from the DVD
3. When the installer loads, select the Utilities menu at the top of the screen.
4. Click Disk Utility
5. When it loads, select the hard drive and how you wish to partition it.
6. When the drive is partitioned, close the disk utility
7. Complete the OS X reinstallation onto one (probably your largest) partition.