① He/she took first place in his/her first national competition.
② They took first place in their first national competition.
「全国大会に初出場初優勝した」
① He/she took first place in his/her first national competition.
※ take first place 優勝する
※ national competition 全国大会
② They took first place in their first national competition.
※ チームなどの場合、複数
We won our first national championship on our first attempt.
「We won our first national championship on our first attempt.」を直訳すると、「私たちは初めての試みで初めての全国チャンピオンシップを勝ち取った」となり、初出場での優勝という偉業を強調しています。
関連する単語とフレーズ:
First-time champions at a national tournament
Victory at our debut national competition
We clinched the national title on our first entry
We achieved our first win at the nationals in our initial participation
アメリカ合衆国 
