今日 is used in informal situation, so when you talk to your friends, just use 今日. If you want to say today, for example, in a business situation, it’s better to use 本日.

''今日''の方が良いと思います。 We use the word ''本日'' in a formal speech.

Not a fluent speaker here, but personally I've never seen 本日 used to refer to "today" I much more often see きょう(今日). I could imagine 本日 being used for 'today' due to the character's meanings, but I've never really seen it used in that way before. But again, take my experience with a grain of salt ;)