Why do you want to know how first revelation came? Will that make the Quran any more true? This question of yours "But I want to know how was first revelation" is the same mindset which created the demand for fancy stories to come in the first place with hundreds of hadiths.
Why can't you just rely on the message of the Quran and not worry about information the information that the Quran thinks is unnecessary to go into?
Believers need to focus on clear verses, get guidance from it and stop worrying about how (a) happened or (b) happened. If the Quran doesn't want you to know that means it ain't important for your guidance!