Hey Dexter,
You can choose either Serial or I2C, using the jumpers on the board as indicated in the reference page. These are two different communication protocols, you can choose the one that is more convenience for you. A trip to Wikipedia will tell you all about them, and there are plenty of tutorials online on how to use them.
As specified in the Arduino Uno page, the board can be powered by a source of 6 to 20V, this means using a 9V battery is OK.