How To Install Kali Linux (or Debian) on Macbook Air (Original 2008 Model)
GUI
You can use the native OS X Disk Utility.app to split, resize, create &amp; remove partitions as desired.
CLI
If you prefer to work with a UNIX-style bash shell, there are tools available via OS X's native Terminal.app command-line interface.

One of these would be a good start:
root@host:~# diskutil
root@host:~# diskutil list
root@host:~# diskutil listFileystems
root@host:~# diskutil activity

Once you have a bit of info, you might want a few of these:
root@host:~# diskutil splitPartition
root@host:~# diskutil resizeVolume
root@host:~# diskutil eraseDisk
root@host:~# diskutil eraseVolume  

Boot from your installation media by holding the alt/⌥ option key during startup.