Hi Cathleen,
I believe you are looking for the Listbuilder tablink, located in the Campaigns tab. Using Listbuilder, you can generate your list of who you want to include in your mass email. If you already have tags setup, you should be able to choose the tag name you want to use, and the Listbuilder will only include contacts with that tag.
I would advise you to look at our Help Center Articles on Listbuilder and Campaigns, they should help explain this is a little more detail. Here is a link for Listbuilder, and here is a link for working with Campaigns. Keep in mind listbuilder doesn't actually add all contacts to your campaign, I believe you would still have to add them manually. If you are required to add many contacts to a Campaign, you might want consider first creating your campaign, then export your Contacts Table, add the exact name of the Campaign you created to each Contact you want to include, then import your list back to Method. If you need some information regarding importing/exporting, take a look at this link. I hope this helps.
-Ben