Let's have a little clarification first. Are you trying to access user data stored by a Google service, or your own service?
I am assuming the first case. Then,
1) You'll need a credential for a
web server. On
Google Developer Console, you can create such a credential for your application(webportal) and configure the oauth redirect url as yours.
2) It depends on whose data you want to access. If it's users', then yes (but they themselves have to register with Google, not you or your application). If it's your application's, then you don't have to create Google accounts for any of your users. Just create a service account for your application.
3) Client ID/secret is an ID of your application. You only have to create one pair, no matter how many users you are going to have.
If you want to authorize users for your own service, then you should probably look at options like
Login with Google, but that's beyond the scope of this forum.