I think what is going to drive you forward is to find something that interests you first and foremost.
Are you interested in graphics?
Are you interested in games?
Do you have a itch to scratch in the area of productivity tools?
Are you consumed by Twitter?
Whatever it is, you'll probably find there are a lot of other folks interested in that too. I think you'll find that if you can focus on something that you are passionate about that has some sort of applicability in the lives of others, and you can take it from concept to a finished, application, there will be a market there.
But whatever you write has to be darn good to get anyone to pay for it because there are a world of free and open source apps out there already. If I am your target market, you'd better wow me if you expect me to fork over my hard-earned greenbacks for your application.
You probably already know whether you can market things yourself, and I'd guess that you aren't a marketing person. No slight intended against you - I'm not one either. If you were, you would have phrased your question differently or not asked it at all. As others have posted, you'll need a good marketing machine to get your app noticed. There are lots of innovative ways to get your app noticed and depending on the platform, there are some easy ways to handle the business side (e.g., the iPhone app store).
If it were me, I'd focus on developing something that is of high quality and give a "lite" version of it away... let the end user try it out and see if it has a market. I've purchased a TON of apps from the iPhone app store by trying the "lite" version first and then forking over my money for the full version.
Focus on quality and the market will be there, I bet.
Just my two cents.