Personally I find that doing whatever seems natural at the time is best for your first play through. You can use the starting classes to help you pick a base, but past that you might find different weapons you want to use that you'll need to level up for. Maybe start with one of the hybrid classes.
You can re-spec in Dark Souls 3, so don't worry too much. Just enjoy it.
Cool. I watched a video to explain the starting classes and the Mercenary seems up my alley. I'm thinking of focusing on Dex for katanas and spears.