Go to Configuration > Integrations > Halo API - here you will see the Resource Server URL and the Authorisation Server URL as well as your tenant name (if applicable)
To start using the API you need to first create an API Application which contains a set of permissions and credentials as well as some other configuraiton options.
The most commonly used Authentication method is Client ID and Secret but you do also need to select an Agent to impersonate. The Agent selected here will also be used for permissions which are further restricted by selecting permissions on the Permissions.
Once this step is complete and saved you can also download examples that can be opened in Postman to help get started