Start at the beginning. Too many SQL users (including developers!) haven't a clue how to properly use it. As a DBA, you'll be called upon to provide that, among other things. So start with the theory and practice of SQL. Especially since it actually is founded upon fairly solid theory, meaning that if you know the theory the practice suddenly becomes a lot smoother. The rest will follow from that.
See db-class.org for a MOOC intro. If you've worked your way through that you'll know where to start looking for learning about the DBA-type things you'll need to do: Schemata, indices, query tuning, and then the subtler tuning like moving tables and indices around on disk or solid state or in-memory or what-have-you. And the basic knowledge will be useful any time a user asks for your DBA-hatted help.
As to exchange, it's crap, and you'll be better off knowing less about its internals. It's hairy and quirky and apt to eat your mail. In fact, it's not even a proper mail server: It's a suitable server for outlook, just as outlook is not a proper email client, but a suitable client to exchange. The combination means a lot of interop trouble that could've easily been avoided.
Since you'll be called upon to make it play ("nicely" is not in the books) with the rest of the world, again, start from principles. Learn how to set up an MTA, know how SMTP and IMAP work. Send yourself an email by telnet. Know what the various headers do. That MTA set up with matching IMAP server, don't have to be exchange at first, in fact it's better not to. Once you know how the rest of the world does it, you can learn how exchange fscks it all up, and how to keep the thing on a leash.
For bonus points, learn how to provide everything that exchange purports to provide ("collaboration" and calendaring and "syncing" and so on, as well as half-assed not-entirely-unlike-email type "messaging") using open-source software. Get that down smoothly (there are several ways and alternatives available these days) and you have another selling point: Providing a better experience with less cost.
That was what you're looking for, right? Points to sell yourself with?