て初（はじ）めて is an expression in Japanese that highlights one verb as being the catalyst for some particular result/state. Verb て初（はじ）めて literally means ‘with (A) done, for the first time (B)’. Often, this translates similarly to ‘only after I (A) did I (B)’, or ‘not until (A) did I (B)’.
-
先生（せんせい）になって初（はじ）めて、先生（せんせい）の大変（たいへん）さが分（わ）かった。
I didn't understand how hard it is to be a teacher, until I became one.
好（す）きなバンドのコンサートに行（い）って初（はじ）めて、歌手（かしゅ）の人（ひと）が男（おとこ）の人（ひと）だと気（き）づいた。
I didn't notice that the singer of the band was a man, until I went to the concert.
In this structure, it should be noted that the (A) verb is not necessarily being done for the first time ever, just that this time resulted in the speaker’s first experience of (B).
-
駅（えき）にある時間（じかん）表（ひょう）を見（み）て初（はじ）めて、土日（どにち）には電車（でんしゃ）が来（こ）ないことを知（し）った。
I didn't know that trains don't come during the weekends, until I saw the train schedule.
Originally, 始（はじ）めて ‘commencing with’ and 初（はじ）めて ‘starting with’ were both used with this expression. However, this is considered incorrect in modern Japanese, as only 初（はじ）めて strictly carries the meaning of ‘for the first time’.
-
カレンダーを見（み）て始（はじ）めて今日（きょう）は休日（きゅうじつ）だってことを知（し）った。
Commencing with checking the calendar, I noticed that today is a holiday.
カレンダーを見（み）て初（はじ）めて今日（きょう）が休日（きゅうじつ）だってことを知（し）った。
Only after checking the calendar did I notice that it is a holiday today.