Your ISP will have assigned you an email address. You launch Claws and click on Configure - Create account. You need to enter information about your internet connection and email service, all of which your ISP provides:
> user name and password, as used to connect to the internet
> your email address
> addresses of the servers to which outgoing mail should be sent and from which incoming mail should be retrieved.
You can find out more about Claws -- how to use, available plugins, creating filtering rules to sort incoming mail, etc -- from