To enable a log in button, you will need two pieces of information from the provider (Google, Facebook etc); a client ID, and a client secret. These will be in the form of a string of letters and numbers–the exact format depends on the provider.
To enable a new provider, go to Edit Credentials pages in the admin site, which you will find under External Logins. There will be a form for each provider there and instructions regarding where to go to register your application. Take note of the callback URL, which will be required by the provider.
Once you have registered with the provider, copy and paste the client ID / client secret from the provider in to the Edit Credentials form, and check the 'Enabled' checkbox. Once enabled, you should see a 'Sign in with…' button for the provider in the standard Sign In dialog.