Start with Dark Souls 1. It has the best world design and bosses.
Also, most bosses telegraph their attacks, so if you die to a certain attack, you’ll know what to look out for next time. Or, you can stay back from the boss and watch its moveset, and then you can understand how to beat it. It’s not that hard when you get the hang of it. Also, learn to parry. Also, if you’re stuck on a part, ask for help here, I know the game pretty well, and Meanlucario knows all three games and bloodborne, and I think demon’s souls.