GCP operates primarily with service accounts and roles. Additionally, and distinct from AWS, GCP has the notion of projects which give you namespaced separation of different roles/resources/etc.
Once you have signed up for GCP, you need to log in here. When you're logged in, you'll be greeted with the dashboard for your default project.
Now that we've logged in, we need to create a temporary user to provision the infrastructure with. Service accounts can make changes on your account as long as they have the proper permissions which are given to them via policies. To create these users, on the left sidebar of the screen, click on the Hamburger Menu, then IAM & Admin, then Service Accounts.
Now you can click on Create Service Account. Fill in the required fields (name, username), then click Next.
To deploy infrastructure, generally the Project > "Editor" + "Owner" permission is required, so you can select that.
On the third screen, you do not need to complete anything here, so just click on Done.
Now that the account has been created, we can create an access key for it. Click on the newly created account to open it's properties page.
At the bottom, you'll notice a Keys section, with an Add Key option. Click on that and then click on Create New Key.
You will need to create a JSON key, so make sure that option is selected and click on Create.
GCP console will generate the key and save it as a JSON file to your computer. You'll get a confirmation prompt that you can now close.