Can we submit an app to playstore that is signed by debug.keystore ?

No..
You can't you have to create a keystore and sign the app using that key.
Make sure the key is safe..or else you can push the update (it will cause signature error) 

how to get the our andoird app signed by debug.keystore ?

1)Make sure you have built the app atleast one time
2)Go to the bin folder and copy the .apk file that's enough.