How to sign APKs

Android Developer page explaining the same thing as below

To test your app on a device you must export an application installer package (APK). The package MUST be digitally signed, otherwise your android phone/tablet will not let you install it.

You can use Google's debug Key and Keystore:

The keyring is located at /home/user/.android/debug.keystore

Hint: Go to window → preferences → android → build, you can see the keyring location there

Keystore password is 'android',

Key password is 'android'

Key alias is “androiddebugkey”

To export the signed application package, in Eclipse right click on the project, select 'android tools' and 'Export signed package'