N3
て初めて
てはじめて
For the first time, Not until, Only after, Only when
先生になって初めて、先生の大変さが分かった。
It was only upon becoming a teacher, that I understood how hard it is to be one.
Explanation
「Vて初めて」 means 'only after doing... for the first time'. Until then you did not notice, understand or manage something; only after going through the first event do you finally realise it or become able to. The second half usually has verbs of understanding, noticing or ability: 「分かる」, 「気づく」, 「知る」, 「できるようになった」. It is often used for life lessons: 「病気になって初めて、健康のありがたさが分かった」 (Only after falling ill did I understand how precious health is).
Mixed up with
「~てから」 simply gives the order of events: 「ご飯を食べてから出かける」 (go out after eating). 「~て初めて」 has the feeling 'I had never realised this before'.
The second half is normally a change in understanding or state, not a plain daily plan. You cannot say 「×薬を飲んで初めて病院に行った」.