So you have picked up a Create 2 to learn about robots but would like to control it from your smartphone? mjrovai has shared a guide to help you get started. For this particular project, an Arduino UNO and a HC-06 Bluetooth module were used.
The guide covers how to connect your Arduino to the robot via serial port and ways to control its motors and other features. Since Create 2 is developed for hackers, you can do a whole lot more with it. More information about this project is available here.