So there are gonna be several links in this post directing you to other places. I don't think it entirely necessary to repeat everything someone else has already written. That said, I'll be giving a sort of high level overview of things that are changing and stuff you should (or shouldn't) do before you get started.
First things first. If you already own Destiny 2, LINK YOUR ACCOUNT! Destiny 2 is moving away from Battle.net launcher and once it is gone it is gone for good. If you haven't linked your account your characters, currency, and progress from you account goes does with it. An FAQ about how this is gonna work is included in that link.
Moving on. If you want some insight into what's coming in Shadowkeep from the Bungie team, it's probably worthwhile to check out this ViDoc. Along with that, you should probably check out their last TWAB (This Week At Bungie) before the release next week.
Alright, let's get to some of the questions.
Q: What do I need to do before launch?
A: Not much really. Paul Tassi at Forbes makes several suggestions here. I mostly echo that post. Since launch is so close unless you really want to grind there isn't a lot that I'd suggest pushing for. Be sure to do the things below.
Link your account
Finish the campaign
Dismantle your Eververse gear (and most other gear as well)
Q: If I don't buy Shadowkeep, will I still be able to play with people who own it?
A: The short answer is yes. If you are F2P or someone that bought Forsaken, this is the best answer.
Q: My power level is like 200. How will I catch up?
A: Don't worry about it! If you are one of the people who didn't hit the gear cap of 750 or max level, it doesn't matter. You'll be boosted to 750 with Shadowkeep's launch and the level up system is going away.
Q: What if I already bought Forsaken but didn't buy the annual pass. Do I still need to buy it since it is all bundled together now?
A: If you already purchased Forsaken but didn't buy the annual pass, worry not! The annual pass is now included for anyone that purchased Forsaken.
Q: Alright Prince, you've made a good case for me playing the game but what about the wonky ass story?
A: Overall, this is where Bungie has struggled most but that doesn't mean the story is bad. Honestly, it's really good and they are getting better at telling it. That said, some people outside of the game are better at putting it all together in an easy way to follow. My name is Byf and Myelin Games do a great job at filling in the gaps in the story that aren't directly told in the game. If you are completely new to Destiny 2, this recap of the Destiny lore is a worthwhile watch (you don't have to watch it all at once)!
That covers most of the big questions that I've seen, but if you folks have more feel free to reply to this post or hit me up on Discord.
Thanks for reading!