Need some help Re: Office 365 management

MyTechSyd 

1/ Someone who has access to their O365 portal needs to create you an account.  I would suggest calling it something like "admin.xxxx@domain.onmicrosoft.com", and include some random numbers after the "xxxx", so that the account name is hard to guess.  Also, make sure you set up 2FA immediately after it's available, to secure the account.

2/ No; you'll login into https://portal.office.com and then use the above account.  That will let you access the management portal for their instance, since your account name will contain their tenant name. 

3/ No; again, after someone sets up your admin account in their portal, you'll use that to manage their tenant in a browser.  There's nothing for you to setup, install, etc.

You might consider taking the 365 Fundamentals and Azure Fundamentals courses, as they're not only a good route to a basic understanding of both platforms, but both are perpetual certifications and show that you at least know the basics of managing both products.