I'm just a mediocre Destiny 2 player who would really like to know how to get the starting quests for the Thorn hand cannon. (Let's face it, Crimson and others are great, but I'd like to say I have it.) I'd also rather get help from my fellow guardians than watch a YouTube video.
Go to the Saltmines in the EDZ. There is a teleporter at the end. Go through the teleporter and walk straight up the hill. At the end is a small tunnel, if you jump up and walk through this tunnel you will reach a campside. If you go to the burned campfire you will receive a prompt to inspect it. There you go :).