Cannot login after registration via Google+

First, we're glad to hear it sounds like you got things setup. As for your questions:

1. How user will know what username and ID after registration he/she should use?

When the user returns, they can simply login with their Google+ credentials. A Joomla account is created for the user as well though that they can use to authenticate with when they return. Those credentials should be automatically emailed to the user. If they aren't, let us know what social profile plugins you have installed as one of those may be responsible for sending the new user email. Is any new user email being sent?

I hope that helps explain, but if you need anything else, just let us know.

Thanks,
Alex