I made an Android app for my school project last semester. If you're thinking about making an app, you should know Java beforehand. Not because Java is difficult or anything like that, but the Android API framework takes getting used to. And you really don't want to be uncomfortable with Java and the Android API at the same time.
The other thing is, you need to have solid OOP skills since Android is designed for running multiple threads concurrently. I can't tell you how many times I had to go back and read about OOP concepts because of the Android API. (However, I was doing some fairly complicated stuff.)
Anyways, that being said, there are tons of tutorials out there that can help you out
You can end up doing some really cool things, and it can be rewarding.