Actually it is "How should I use?" It would be used casually only.

It should be fine to use on both formal and casual levels. FYI = for your information FYI, if you are unable to pass the exam, you will have to retake the whole semester. FYI, if you are going for a bike riding with us, please make sure that you bring enough water along.

Hm... I don't know japanese, but I'd never use an abreviation in a business letter. Fomal languaje in english has a very strict structure and getting out of it means your use of english is not good, despite of the level you may have. My irish teacher said "at least write the hole thing. You can't say things like OMG in a letter, or some people may not understand you" - as I first didn't understand what you meant with FYI. In business languaje saying "to inform you" "you may need to know..." is much more appropiate in my opinion. Hope it was of any help.

Thanks! I understand it is casual.

I would not use this in a formal letter or for work. Usually casual, like a text message (SMS) or between friends.