How it works is like this.
At level one of your character you get your max hit die plus modifier.
Every level after that you roll for hp. What you rolled on your hit die plus modifier.
So if you multiclass and start with warlock you get 8+con
Then lets say you class sorcerer, then you roll the D6 and add modifier, then another level of warlock. You roll the D8 and add modifier.
You only get the maximum roll on your first character level.
As far as I know at least.