Which article should I use in front of a French female secretary?

Use the feminine article. If it is true, use la (les) instead of une(des).

For example, there is only one female secretary in the school. If you want to say that the secretary sent me.

The secretary told me that everything was fine.

If there are several female secretaries in the school, you should say that the principal criticized a female secretary, Le Directeur a Critique une Secré taire. Criticized all female secretaries ... comments on secretaries. Criticized some female secretaries.