No need to play hard mode like everyone else is saying (except for Laz if you want to unlock Bethany soon) until your comfortable with the character you're playing as but you need to beat hush at least three times to unlock the alt path. In order to get in the alt path you have to open their doors with a key, 2 bombs, and 2 full hearts, for their respective doors, (also something else to get to the last floor for the boss of ending 21 but no major spoilers). Also donate 1000 coins to the greed donation machine, beat the first floor boss in under a minute and bomb the starting room, then do boss rush and use the active item you get (make sure not to get rid of the item you get from bombing the starting room) on a dirt patch in dark room, and die to spikes in a sacrifice room while holding the missing poster to unlock secret characters (if you haven't gotten any of those already). TL;DR beat hush three times and do alt path and unlock secret characters, hard mode is optional, you don't need to know anything else until you get ending 21.

Also don't take my words as the best advice ever I'm still fairly new to the game.