первым долгом

см. тж. в основном; в первую очередь; во-первых; прежде всего

First and foremost, Britain has vast energy reserves.

What they did first of all (or first off) was to call the local geologist.

The final acceptance of this interpretation will depend in the first instance (or place) on confirmation of their results.

Источник: Русско-английский научно-технический словарь на Gufo.me