You need to be proficient in a programming language such as C, C++, Objective C for writing applications on Ios (iPhone, iPad) or Java (Android, Blackberry OS). You should understand the basics of UI design and also the basics of application design, development and testing. Once you have the basics then you have to choose what platform and mobile operating system you want to specialize on and focus on it.
To become a Google Android developer you have to be proficient in Java and learn Android development. Android development can be done on Mac OS-X, Windows or Linux environment.
Along with computer science degree check above what are the other requirements.

Good Luck.