How do GMs typically learn how to run the game? I read the source book and I get ridiculously overwhelmed with the amount of information. I'm not sure how to memorize everything in the source book, mostly stuff involving combat and statistics. I'm usually not a very statistic oriented kinda gal, but my friend group has been desperately wanting to play CPR for months now and I've finally decided to say "fuck it" and make it happen. Any advice from experienced GMs? How do I make this happen?

(UPDATE: I just wanted to say that I appreciate all of the advice in the comments! I haven't responded to every single one of them, but I've definitely read all of them. This girl has zero experience with any TTRPG, hell I haven't even been a player before, but I have the confidence to move forward with this campaign after reading the comments. I've got a good headstart now, and I'm fine with potentially fucking up here & there. Hoping to give my players an awesome experience. :> )