How to add condition for 1st Iteration in views-view-fields.html.twig file
If your code snippet is within a for loop, you could use either loop.first or loop.index in your if statement to target the first iteration. 
 Refer to https://twig.symfony.com/doc/2.x/tags/for.html#the-loop-variable

Thus your twig template would probably resemble:

{% for item in items %}
  {% if loop.first 1 %}
    {% set tag = "active" %}
  {% else %}
    {% set tag = "" %}
  {% endif %}

... render row/field with class in template ...
{% endfor %}