After you subscribe to an API you can find your API Secret on the subscription page along with analytics, pricing and realtime usage information.
Subscription payments are taken monthly from your account. You can view your current bills and invoices in the account billing section.
API usage is recorded in realtime.
The key used to authenticate you to the API servers. This is usually passed through the Authorisation header, however this depends on the API provider. You should be able to find information on how to use your API through their documentation.
Your Subscription Secret should be kept safe as it allows you to be charged for your usage of the API.