To get started, here are a couple of links for you:
Android Developer's Guide
Android Developer's Main Site (With a link to the SDK)
Now, this requires some reading up, and when you do start understanding it, it requires a significant amount of writing of code, which spans knowledge of xml, java, and the specific Android language. A learning curve, but not impossible.
However, if you really aren't affinite with programming, that's fine too. There is another option for you, but it isn't as powerful or versatile as manipulating the hard code yourself: App Inventor
App Inventor requires you to sign up for the program, and it takes some time for you to get registered for it, however it comes with step by step tutorials to get you up and running fast.
Now that you've seen these, get out there and get developing!
Also, here is an app that I personally created: The Tip Calculator. Download it to your computer and transfer it to your phone, and then find it in your phone's file browser, and install it. Enjoy!