This is the easiest step.
Just tell the reader what to do next (ideally buying your product). And be VERY specific about it.
For example... click this link, then click the big green button at the bottom of the page, then fill out your information to join.
NEVER leave anything up to interpretation.
Now IMHO... if you want more sales, you should be plugging your products or services in every email.
Or at the very least leading your prospect down a path to buying.
Here's an example: