Android is one of the major contenders that leverage the portability of mobile operating systems that are used by various mobile manufacturers and many corporate applications. Android have captured the world's smart phone platform mobile operating system. This application is also used by many other mobile manufacturing organizations.

Known as AOSP, or Android Open Source Project, the Android Operating System is the most widely used OS in the world as of today. It covers more than 50% of the total market share. The reason behind such a widespread popularity is the nature of the operating system being Open Source, thus enabling developers to exploit it to the core. Apart from this, Android is the only OS that can be implemented in other embedded systems as well.
Some of the key features of Android Operating System are:

It's open source.
Android OS has rich UI features
Availability of OpenGL Libraries, which enables developers to embed 2-D and 3-D graphics into the user interface

Thus, developing applications on Android attracts developers like anything. Apart from that, since a major portion of the world's smartphone users have Android OS, developing Android apps gives them a huge user base, and thus they can reach out to greater number of people as compared to the developers of other operating systems.

Since Android programming language is more or less like Java, prior knowledge of this language is thus a big plus point. Also, if you know how to create and manage XML files, the designing stage of the application will get simplified, as you can easily declare UI elements in the XML file.
Before you head towards development, make sure that you have the following in your system:

Android SDK
In case you use Eclipse IDE for development, download the ADT plugin
Update your tools using the SDK Manager

Once you are done with setting up your system, do a research on the Play Store, the place where they put all the Android apps to download from. Search for the type of application you want to develop. Since Android is nowadays developer's most favorite OS, there's quite a high chance that you'll find the app! In case you don't, start development ASAP. But if you do, download it and look for the loopholes and flaws that app might be having, eliminate them in your app.

Start developing the app now. The basic components of an Android app are:

Activities: It is a user interface component, corresponding to one frame. An activity returns a value.

Intents: Intent can be considered as an action, or a verb. For example, €call€, €view€, etc are the intents used to direct what action needs to be taken.

Broadcast Receivers: It receives the intent that is broadcasted by any function.

Services: Various pre-defined modules or in-built apps, such as music player, network connection, etc. The
services can be embedded in your apps.

Content Providers: This API enable you to access the content on the device, such as the address book, picture gallery, etc.

Upon integrating the above mentioned components in your app, test it on the emulator in Eclipse IDE. Pack the final version, into an apk bundle, and upload it on the Play Store.