Ask HN: Professional Android development

I'm a hobbyist Android developer and I'm capable of building mildly complex Android applications. Recently I made an app as a freelancer for a client and they want me to work with them remotely (me being the only developer). Now since this is a part-time job I want to be serious about it, and maybe turn it (freelancing) into a primary source of income after I graduate from college.

Although I'm quite comfortable with Android development (including working with 3rd party libraries, client-server model, etc) I want to know how to turn Android development as a hobby into a professional skill.

How did you go about doing it, what challenges did you face and what should I be careful about?

Thanks.