Lads, I need help.

I've been playing Black Crusade with some friends and I have to admit I'm not having as much fun as I think it could be.

I think the DM may not be that great. I will spare you the details but basically I'm thinking about (politely of course) offering to try my hand at it. It's just that I've never done it.

I'm happy to be patient and wait for us to finish up at least this campaign, which at this rate may take a year or two at least but still.

I'd like to prepare as much as possible in advance to be a great GM.

I understand that A LOT of what a GM does is basically improv and that whatever you think should be logical will not be done by the players. But other than that, what tips would you give to a guy trying out as a GM with the FFG system?

I think Dark Heresy might be better because there's more latitude, since you don't have to play the "bad" guys unlike in Black Crusade and you can have a simpler narrative system but I may keep BC for the sake of people already knowing (kinda) the rules.