Creating tags from fields
This is possible with CiviRules.  However, I'd argue that in most cases you'd rather use a Smart Group.

If you're committed to using CiviRules though:


You'll need two rules.  One with a trigger of "Any contact is added" and one with "Any contact is changed".  If this only applies to individuals, you can substitute the appropriate triggers.
The condition will be "Field Value Comparison".
The action will be "Add tag to contact".


Note that you may want another rule to remove the tag if the field is changed and the condition no longer applies.