I would say that you have 2 massive players in the app world (I could be wrong though). That is The Google Play (aka:Android Marketplace) and the App Store. Then you have the amazon marketplace (which I have no experience with so I can't speak for that market).
Android
This is a great place to start testing the waters of app development because of the following:
- Free to submit apps
- Apps hit the market place incredibly quickly after you push them to the marketplace
- Can develop apps on any machine
- You have to market your app and get it out there in front of your audience from the get-go
- In September they had their 25 BILLIONth download (not sure where this is at today)
Apple
The app store is another great place to put apps out there but you have to pay to play; But it does have a few advantages that the Android marketplace doesn't; Although this may be changing with the new app store designs.
- $100 To submit apps to apple
- Must have a Mac to develop (I'm sure that there is a hack to this somewhere, but they really push for you to use a mac)
- When your app passes the review process (typically 5-10 days) your app will go into a "What's New" section which generates downloads and visibility for a short time for your app
- Apple recently celebrated its 50 BILLIONth app download
As far as apps to create. It is just like IM; Pick something that you are interested in and build it. Don't do a flashlight though because there are a million out there lol.
If you are looking to promote other peoples apps check out sites like:
http://greatapps.com they contacted me a little bit ago and had a price point of $500 for them to put my app on their website for a year. So there are ways to promote other peoples apps you just have to add your own spin. Good luck.