Step 1: What is ROS ?
Step 2: The build ...
1. 2 dual-core computers. 1.6ghz or higher with at least 2gb of ram is recommended.
2. A Turtlebot or Turtlebot compatible robot ( http://www.zagrosrobotics.com/shop/custom.aspx?recid=60 )
3. Gyro power/board - optional - ( http://store.iheartengineering.com/robots/turtlebot/accessories/ihe-0200-0000-fa00.html )
3. Wifi router
4. And a 4gb usb flashdrive