When you first create a client, their password is not set.
When you are setting up the booking proposal or creating the shoot, there is an "Email Link to Shoot" toggle. Enabling this will send a link to your client for them to create a password once the shoot is booked. Once they've created their password, that is their password for their portal.
If they forget their password, they can click the "Forgot Password" button on the login page for their portal and they'll be emailed a link to reset the password.
If you'd like to override the password and manually set it (or reset it) for them:
Edit a contact.
Click the "Client Actions" button in the bottom-left corner.
Click the "Set/Reset Password" button.
Set the password.
Note that after you set it, you won't be able to see the password again. For security reasons, we never display the client's password in plain text after it's been saved to their account.
To read more about setting your client's username and password, view this article.