first of all, congratulations for your offer! I think a good preparation would include technical, communication and goal setting/stress management skills.
- On the technical side, quoting a previous post, Excel will be the most important technical thing to master at the beginning, in particular for what concerns VLOOKUP and Pivot tables; an additional useful review may concern PowerPoint, which you will also use pretty intensively. At BCG we got learning courses we could use to improve on them, and I guess you will receive the same at McKinsey, but so far that you have already mastered the skills before joining (you can find several courses online for both for free), even better.
- For what concerns communication, a classic on the topic is the book “How to win friends and influence people” by Dale Carnegie. In case you have time for an additional job before McKinsey, an internship in a sales role could also help a lot.
- Finally, for goal setting/stress management I would recommend “The Secret” by Rhonda Byrne.
A couple of things that could help you during your first weeks are instead the following:
- Take notes when your manager tells you something – this will help you to remember details and will show you care about them to the team.
- Ask for feedback every two-three weeks – this will show you are proactive and willing to learn.
- Always double check. First impression is very important in consulting: if you show you are reliable from the beginning, you will create a reputation of a reliable person.
- Ask for help when you don't know what to do – better to let know you are in trouble with meeting a deadline then missing the deadline.
- Be social and respectful with the support staff – these people are great and influential as well in the company.
You can find some additional information on the topic here:
Hope this helps,