asus and linux (from usb)

First you have to figure out what key to press while the laptop is booting to get into the bios. It differs from laptop to laptop. Then you have to find the setting that controls the boot order and change it so, the computer boots off the usb first. If Ubuntu is installed correctly then it should boot.
 
Ok it's not too painful to do :) Once your in the bios you should be able to scroll left to right using the arrow keys. As you scroll through the bios there should be a section called "boot priorities" or something similar. From there you should see a list of devices that your laptop can boot from(hard drive, DVD player, etc). Usually you should see your hard drive being the first bootable device and then you should be able to click a key that changes its priority. All keys and what they do are listed on the bottom of the screen.