I'm not telling you, so whatcha gona do ...?!

It's a lazy, squashed-up version of "what are you...?" "Watcha doin?" = what are you doing? It's good to know, but really avoid using it, especially in writing. Using slangy and squashed words like this is a very quick way to appear dumb, childish or comical.

whatcha is slang version of "what are you" or "what did you" example: whatcha say? --- what did you say? whatcha doing? --- what are you doing?

what's the meaning of "Whatcha"? what does "Whatcha" mean? http://dj.iciba.com/whatcha/