Building your own robots is a great way to learn about technology. ASCAS has shared a guide that shows you how to build a life-size smartphone controlled BB-8 droid. He used an Arduino Uno, 1 x motor shield, 1 x HC05 Bluetooth module, battery, 1 x inflatable beach ball, and a few other parts to get started.
The above video shows what this droid looks like. More information is available on Instructables.