第一(の/に)
Usage
第一 modifies either verbs with に or nouns with or without の, and means "first". It is also used as a predicate with だ or です. "First, first o all, to begin with; the first; the primary"
Examples
家に帰って、いつも第一にすることは、Eメールのチェックだ。
The first thing I always do when I get home is check my emails.
試験に合格できた第一の理由は、覚えた言葉や漢字が全部試験に出たことだ。
The primary reason I was able to pass the exam is that all the words and Kanji I had memorised were included in the test.