I've only just started using Trello but I've been told I can add an email link into the card. How do I do this?
If you just type in the email address in a comment or any other text portion of the card, it should be automatically displayed as a mailto link which will open the user's default email handler.
You can do this using an email address or using Gmail with Zapier.
This Zap for email to Trello gives you a unique email address like firstname.lastname@example.org and then you can send email to that address which will create cards in Trello.
Alternatively this Zap for Gmail to Trello creates cards in Trello from emails you receive in Gmail that match the labels or filters you assign in the Zap.