You need to create your own signed key in order to publish app. It is very simple and can be done with Eclipse.
Just follow this quick guide:
- Right click on your project in Eclipse -> Android Tools -> Export Signed Application Package
- Click Next
- Select "create new keystore", choose location, enter password
- Click Next and fill needed fields. Write 25 years in validity. (it is the minimum)
- Click next and chose where you want to place apk for Google Play
Thats all. Next time when you want to update your app just select "use existing key" in step 3.