How do I obtain OAuth client ID

I’m creating a Application and want to add Sign in with Google. The documentation says I need my client ID, but google didn't give me one.

As per the documentation, you first go to the GCP credentials page. You can then click on Create Credentials in the top set of actions:

Select "OAuth Client ID" as the type of credential to create, select "Web application" as the application type, and fill in the rest of the prompts. When you've created the credential, you'll be presented with the OAuth client ID.