I am going to write about the process of setting up Ubuntu as a dual boot option on your MacBook. Reading several guides, I decided that the actual process is much easier than you would thougt.
First of all you have to choose an Ubuntu version. [Ubuntu 12.10][Stable] is the latest stable release and the best option for beginners. If you feel as experimental as me you can go with [Ubuntu 13.04][Beta] as well. It's probably buggy and you will encounter issues when fighting with software version, but you will get a bleeding edge version with the newest changes and better drivers. Whatever you choose, start downloding the 64bit Mac version now. It will take some time.
Your MacBook does not boot other OS out of the box. To do this you will need an Bootloader as [rEFIt][EFI]. Download and install it. Just use the installer, no need for customization here.
Next you will need to make an partition for Ubuntu. Use DiskUtility to create a new Partition on your Disk. Don't