て初（はじ）めて is an expression in 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)’.
I didn't understand how hard it is to be a teacher, until I became one.
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).
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’.
Commencing with checking the calendar, I noticed that today is a holiday.
Only after checking the calendar did I notice that it is a holiday today.