You hold down the Option key while booting.
> Or explain how to dual boot a Mac with OSX and Linux
Install rEFIt (comes in a nice, standard OS X installer package), use the nice GUI Bootcamp or Disk Utility apps to make a partition, insert the Linux CD, reboot while holding down C.
If you're incapable of following these instructions, I seriously doubt you can manage to boot from a USB stick or dual-boot Windows and Linux on any typical PC.