Congrats on your offer!
Since there is already some good advice posted by my fellow experts I thought I'd give you my perspective as well.
When I got the offer some years ago I did the same. I reached out to people I knew in McKinsey and people who interviewed me to ask: what can I do to make the start easier? how can I prepare?
The answer from everyone was: Relax! Enjoy your time before you start and don't think about it. You will figure it out on the job. I followed that advice and it made sense to me once I joined.
When you start at McKinsey there are 2 ways to learn:
- Formal training. The formal training sessions/ weeks/ days in the beginning, are nice, however, they are more for networking and meeting your peers. You learn some interesting concepts and get some useful tips from more experienced consultants BUT
- Constant and implicit learning on the job is where it's at. No matter if you are a newcomer or a veteran after 2 years, you will always find yourself on a steep learning curve. As soon as you barely mastered one skill or the skills needed for one level in the hierarchy, you will take care of things, which are expected from a more senior colleague. This cycle never ends. You are expected to learn on the job, learn from your colleagues, your mentors, sometimes even the client. So basically a newly promoted Engagement Manager has the same 'struggle' as a new-hire Business Analyst. They both need to work in a completely new environment and role.
Knowing that, if we now go back to square one in your McKinsey journey it makes perfect sense to enter the firm with a blank slate with a lot of curiosity and eagerness to soak it all up and quickly learn the ropes.
No book, no training, no coach can prepare you for your first day, your first week, your first engagement. Nothing matches the experience and the learning and this is a good thing (also the reason why ex-McKinsey are valued highly on the job market).
You will learn everything you need to master while doing it. You will be thrown in the cold water and need to swim. However, your colleagues will always be happy to help you and mentor you. And for the rest, you will figure everything out along the way. The key here is always to ask for tips, shortcuts, feedback, etc. Don't be quiet if you get stuck.
Also, for every technical problem (IT, Excel question, etc) McKinsey has a Global Helpdesk and the rule is to call them for every problem you can't solve within 5 minutes. They will fix your computer, guide you through Excel formulas, etc.
Lastly, if you have no domain knowledge about a certain industry or topic, read through the internal library of documentation (which is endless) and call some of the firm experts on the topic. Usually, they are happy to offer you a short call to get you up to speed.
Don't sweat the small stuff and enjoy the ride!