The Arduino Uno is battery powered by making up a battery clip with connector. A battery clip is soldered to a 2.1mm barrel connector (centre positive). The battery holder with 6 cells will provide about 9V with ordinary 1.5V cells or about 7.2V with rechargeable cells.
Find the article for this video at: http://startingelectronics.org/articles/arduino/battery-powering-arduino-uno
The Starting Electronics website contains information on how to start electronics as a hobby: http://startingelectronics.org/beginners