YOU SAID:

I do not know who is on first

INTO JAPANESE

私は誰が最初にいいのか分からない

BACK INTO ENGLISH

I don't know who's the first to do it.

INTO JAPANESE

私は誰がそれを最初に行うべきか分からない。

BACK INTO ENGLISH

I don't know who should do it first.

INTO JAPANESE

誰が最初にやるべきか分からない。

BACK INTO ENGLISH

I don't know who should do it first.