Some find this a usable way to control booting between OSes.
On installation, Debian sets GRUB as the default boot manager and boot loader.
Turning on the computer will start GRUB, and start Debian.
OS X can be booted by holding down the Option key at power on and entering OS X from Apple's Startup Manager.
To run Debian, you will want two partitions: one for the Debian OS, and one for swap space for Debian. The network install is a minimal image containing few packages.
You will shrink the OS X partition, so decide how much space you want to give your OS X and Debian partitions. Other other packages are downloaded as needed from the internet.
This works well if you have a wired ethernet connection.
As new Macs don't have an on-board ethernet port, an ethernet-to-thunderbolt adaptor will be required and will work during the Debian install process.
Get the amd64 image which works with Macbook 2007+ architecture.
Mount the image onto a mountable physical volume (CD, DVD or USB stick).