Boot options for Macbook

You want to try ubuntu or want to boot from live cd. here are options.

Products Affected
MacBook Pro, MacBook Air, MacBook, iMac, Mac mini, Mac Pro
Keystroke Description
Press C during startup Start up from a bootable CD or DVD, such as the Mac OS X Install disc that came with the computer.
Press D during startup Start up in Apple Hardware Test (AHT).
Press Option-Command-P-R until you hear two beeps. Reset NVRAM
Press Option during startup Starts into Startup Manager, where you can select a Mac OS X volume to start from. Note: Press N to make the the first bootable Network volume appear as well.
Press Eject, F12, or hold the mouse or trackpad button Ejects any removable media, such as an optical disc.
Press N during startup Attempt to start up from a compatible network server (NetBoot).
Press T during startup Start up in Target Disk Mode.
Press Shift during startup Start up in Safe Boot mode and temporarily disable login items.
Press Command-V during startup Start up in Verbose mode.
Press Command-S during startup Start up in Single-User mode.
Press Option-N during startup Start from a NetBoot server using the default boot image.

original link

http://support.apple.com/kb/ht1533