It sounds like you need a Trackable Link.

Trackable Links allow you to associate an Action Set with a link in an email. When somebody clicks it, the system knows who they are and applies the Action Set to their person record. You could have the Action Set simply Tag them, or put them in a Follow-up Sequence, or even create an Order. The possibilities are endless.
But in this case, you want to Tag them and shoot off an Email. The steps to accomplish this are:
1. Setup -> Templates
2. Edit or Add an Email Template
3. click Open Merge Window
4. click Create Link on the Misc tab
5. fill in the necessary information
6. click Save
7. unfortunately, that closed the popup so navigate back to the Misc tab and [Edit] the Link you just created
8. there is now an Actions tab, go there
9. add an Action that will add a Tag
10. add an Action that will send the Email
11. Save that
12. merge your Link into the Email
13. Save the template
14. Send it out!
Hope this helps.