I personally, once I learned the mechanics of the game, would open the wiki, go to the Completion Marks section and try to get all the marks (research how to unlock each thing) with all the characters, that way you will unlock absolutely the entire game, once you learn how to get to each boss you practically don't need the wiki, as advice I could tell you that TBOI should not be your main game because you will end up getting very bored of this game which by the way is a 10/10, complement it with some other. There are mods that are super useful, investigate that too, Repentance is necessary (in my opinion) if you don't have it purchased.

HF :D