There is a debug key to sign our apps and running them on our mobiles...
But we cannot publish the apps signed like this..
Follow these steps to export a signed app:
1)File->Export
2)Android->Export And....
3)Provide the name of the project.
4)"Use Existing keystore".
5)Location = C://...Users/....your_name(or Student)/.android/debug.keystore
Password = android
6)Alias = androiddebugkey
Password = android
7)Finally select Destination..