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 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 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. 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 give it a format, leave it as free space. Choose anything up from 5GB, Ubuntu can handly that. I would consider 20GB as the minimum to use it for more than a pretty screensaver. Apply the partition, wait a few seconds and you are done.
Your Ubuntu image is downloaded. That's fine. You have a .iso file, which is a disk image. We need to write it on the USB stick. It has to be 4GB FAT32 formated drive. Identify it's device name by typing diskutil list in your terminal and coosing the device which matches it's space. The device name looks like /dev/disk1 or similar. Now write the image on the drive by typing sudo dd if=Downloads/raring-desktop-amd64+mac.iso of=/dev/disk1 bs=1m Where 'if' is the input file, the image you have jsut downloaded, and 'of' the drive you have just identified. 'bs' is blocksize and can be adjusted if you like. The is going to take some time, for me it finished withing 15 minutes or so.
Congratulations, you have nearly installed Ubuntu. Now comes the interesting part. You can reboot now and should be greeted with the rEFIt bootloader which lets you choose the OS to boot from with your arrow keys and enter. It should have detected Ubuntu on the USB stick and lets you boot from it. Now follow the install instructions from Ubuntu. Counterintuitivly, this is the easy part. Ubuntu will detect OS X and offer you to install it as a dual boot option in the free partition. You are done. Welcome to Ubuntu