What do I need to know to start developing for the i'm Watch?
The i'm Watch runs a custom version of Android, called i’m Droid, which is optimized for the watch form factor. It is, though, mostly API compatible with the standard Android SDK. In order to be able to write apps for the i’m Watch you only need your standard Android development skills, which include knowledge of the Android SDK and the Java programming language.
The most used Android development tool is Eclipse, one of the IDEs that Google itself has chosen for Android, by leveraging Google's ADT plugin. Google has also created the Android Studio, a new IDE based on IntelliJ IDEA and focused on Android development. You can also use IntelliJ IDEA, or any text editor for Java and XML files, even command-line editors such as VIM, in conjunction with Android's build tools. You can also use an AVD (Android Virtual Device) to test your apps if you still don't own an i'm Watch. You can find all the configuration details on the Resources page.
Which OS do I need to develop applications for the i’m Watch?
To develop apps for the i'm Watch, you can use any of the OSes supported by the Android SDK, which include all the major OSes: Windows, Linux and Mac OS X. You will of course also need a supported JDK (Java Development Kit) and Android SDK ready on your computer.
I'm new to Java. Where should I start?
You can find guides and tutorials on the Oracle website. Oracle is the company that mantains and develops the Java platform, originally created by Sun Microsystems (later acquired by Oracle).