I'd seriously consider being formal when money is involved. This will not only protect you and what you'd be being paid, but also the person who would be utilizing your services, as they would, as Danger mentions, have an increased sense of trust in your work. The formality will show them that you fully plan on completing whatever your set task was.
Barring that, if you're exploring mobile development as a hobby, consider writing up an app that you feel has a targeted use for more than just yourself and close friends, and making it public. Whether it's a tool or a game, ensure it's got value to the broader public, then do as your initial thought was, and market it. You could also take on personal projects for friends, provided they're willing to pay you and you're good friends with them, and ensure you meet the goals for the project in a timely fashion.
This should really get your foot in the metaphorical door of mobile development and allow you to solicit feedback about your work, and it might earn you some money along the way if you choose to monetize said app. You should be able to determine from this point if you'd like to start making more formal strides as a mobile dev freelancer, and if you do, definitely start approaching it as professionally as possible. When money is involved, and it's not a deal between friends, you should be certain to be as professional as you can.