Assuming you haven't done it yet, look at the 3 classes as pick the one you think is the coolest or is the most appealing you you individually. Use the boost on that one character, then focus on it until you are a fairly high light level (maybe 280-290 even). Then when you start using new characters, take the unwanted, high light head from the first character and do something called infusing. It lets you transfer some light from better gear to lower gear (even if they are for different classes). That way your other characters start off better and it makes the game easier. Sorry for the wall of text and welcome to the grind.