Same advice as to someone starting into a new career path of sysadmin-ing : Read "The Practice of System and Network Administration" and make sure all points are covered.
Work with all stakeholders. The business, application users, existing IT staff, DBAs, networks. Find the gaps, and find out if you just haven't asked the right questions to the right people, or have to log into the systems and find the answers for yourself.
Start taking notes, writing hierarchy diagrams, and get going. I use OneNote, but anything similar will do. So will a plain text file, but some bells and whistles are nice. Without spending a crap-ton of money on discovery tools, you can't automate most of this, and you can't automate all of it without knowing it already, unfortunately.
Count yourself lucky it's only two racks: that's nothing.