Starting Forsaken with new updates

You can't just click on Tangled Shore to start?

I highly recommend checking out the video below by Houndish. He does a great job of explaining how to obtain Quests for New Light Players, and those returning to the game after a break. Though the short answer to your question is you should check in the Tower with Amanda Holiday. I am currently updating the Information and Leveling Guide, but there is a lot to go through, so it might not be fully up to date for a week or two.

Is there a guide anywhere I can check out to see how the weapon mods and stuff work? I played the hell out of the base game when it came out but fell off before the first expansion. Logging back in but I’m totally lost on what do do with my weapons and armor all being 750.

@fleasquireInformation and Leveling Guide, see Weapons/Mods/Armor/Masterworks sections. There is a video by Datto linked there explaining Armor 2.0, and I tried to explain Weapon Mods, though it's a pretty simple concept. You unlock Weapon/Armor Mods as permanent unlocks through activities or the Gunsmith. They cost $5k glimmer to equip each time, but are infinite equips once unlocked. Here is a list of all Mods and a short description of what they do.