You can't just jump in and commentate competitive matches without building some experience first in actually commentating competitive matches. And I don't think you've ever commentated any matches...
BUT, that's an easy fix. Keep a close eye on MRBC, that's a great opportunity to get casting experience, since there are so many matches being played at different levels.
Watch some previous casts here:
https://mrbcleague.com/casts
Learning the competitive meta and being able to explain it to the layman is a big deal for casters, and it will take a while to build up that knowledge. But thankfully, that's something that can be built up over time. Charisma, energy, and enthusiasm isn't really something you can build, but you already have that, which is perfect.
I look forward to seeing you getting your feet wet in MRBC. If you livestream your matches (which I would advise you do), don't forget the obligatory 5-minute stream delay, and remember to enable Past Broadcasts in your settings, so that your streams don't go *poof* never to be seen again.