The first thing that got me interested in English was music.
The reason I first became interested in English was music.
「英語に興味を持った一番最初の理由は音楽です」を英語で表現するなら、一番一般的な言い方はThe first thing that got me interested in English was music. さらに、「英語を勉強すること」や「英語を学ぶこと」をもっと具体的に表現するなら、The first thing that got me interested in studying/learning English was music.とも言えます。何かの「きっかけ」を語るときは、このThe first thing that got me interested in〜の表現を使うのが便利です。
なお、「理由」は英語でreasonと言いますので、The reason I first became interested in English was music.とみ言えます。
The main reason I became interested in English was music.
What got me interested in English was music.
ご質問ありがとうございます。
・「The main reason I became interested in English was music. 」「What got me interested in English was music. 」
＝「英語に興味を持った一番最初の理由は音楽です。」
（例文）Why are you studying English?// The main reason I became interested in English was music.
（訳）あなたはなぜ英語の勉強をしているんですか？//英語に興味を持った一番最初の理由は音楽です。
（例文）What got me interested in English was music. I love rap music.
（訳）英語に興味を持った一番最初の理由は音楽です。私はラップが好きです。
お役に立てれば嬉しいです。
Coco
アメリカ合衆国 
日本 
