I am going to assume you mean to run backtrack as a vm. Thats actually straight forward. You will need to install virtualbox. Virtualbox will let you run the backtrack4 cd in a "virtual machine". However, if your talking about the actual tools included in backtrack4 i would suggest that you comp[ile a list of applications, then, searching the ubuntu repos for them. I have a feeling that if you only want the TOOLS used in backtrack, you may need to compile from source and get hold of some oddball deps. That has been my experience.
1. READ UP HERE.
2.THIS SHOULD GET YOU INSTALLED AND READY FOR YOUR FIRST VM.
sudo apt-get install virtualbox-ose virtualbox-guest-additions virtualbox
Hope this helps. You might want to read up on virtualbox a bit before run ning off and installing though. While the GUI makes it easy, it always helps to understand whaat your doing. As far as backtrack goes, it not like a magic Hack tool...just so you know. Your gonna need to know how to use and configure the tools. My experience with backtrack was this: all the tools in once place, already compiled, waiting for config and user direction. Any way, have fun.
EDIT: After googling "adding backtrack repo to ubuntu" I found this. Google is your friend.
However, from personal experiance, I would focus more on getting used to linux, networking, and the CLI in general. If getting packages installed is proving to be an issue...configuring things like kismet and madwifi will be pure joy for ya. YMWV.