Mobile apps -- huge audience and very limited scope of the apps. Take the next best idea, get a designer for a few hours for design and images, spend a week or two on development and put it into the Appstore. It's a lot of fun too! I further recommend releasing for both iPhone and Android -- once you have the idea, the algorithms and the design, it takes very little extra effort to reach the other platform.
For one of the first steps: register a domain, setup email and a wordpress with a nice theme, start blogging and have all your future apps be part of your 'business'. You can flexibly expand from there.
I'd skip iPhone and just focus on Android, mostly to save time. Browse around the iPhone app store though and find some corny apps like a fake fingerprint reader. "Steal" that idea and build an Android equivalent with mobile ads. After your first couple of apps, you could probably crank out a new one in part of a weekend.
I don't know if time spent blogging about and promoting your apps would pay off when instead you could be building more apps or possibly improving the existing ones that are getting good download numbers.
For one of the first steps: register a domain, setup email and a wordpress with a nice theme, start blogging and have all your future apps be part of your 'business'. You can flexibly expand from there.