第一

The word 第一 can be used with the particles の or に to modify nouns or verbs. It's generally translated as "first", "first of all", or "to begin with."

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 number one reason I was able to pass the test is that all the words and kanji that I studied appeared on the test.

リーさんは猫と犬の健康が第一だと言ってたんだ。
Lee-san said that the health of his cat and dog comes first.

彼女の第一印象は悪かったのに、なぜか私と彼女はとてもいい友達になった。
Even though my first impression of her was bad, somehow her and I became really good friends.

週末時に、いつも第一に食べる食べ物はラーメンとお菓子だよね。
When it's the weekend, the first food that I eat is always ramen and sweets.