While I know 40k, and I've played 40k RPGs, I haven't played Dark Heresy (it's way too mechanics heavy for me) so I can't help you there.
I can only offer you general GMing advice that I find useful.
Keep things moving. Fast.
Stay on the same page. If someone is going against the tone of the game, or is stupid, or they've misunderstood something, then don't just carry on awkwardly. Check in with folks to raise the issue, immediately, and don't make it a big thing, and then carry on.
Say yes or roll the dice. Whatever the players want, give it. (Unless it is stupid, see above), don't block or quibble or delay. Unless it is interesting for them not to have it, in which case go to the dice.
Cut to the action. Action doesn't have to be a fight, but it has to be interesting. There's no need for travel times and loading screens in a table top game, so don't use them.
Roll the dice ONLY when it is interesting. And it must be interesting regardless of the result, pass or fail. So, if there is vital info that would be totally boring if they didn't get it, then don't make them roll to get it.
You are just one player. Use the others.
Bring fun and enthusiasm yourself, but remember that -their- fun isn't -your- responsibility. Demand fun and enthusiasm from your players too, and use it to fuel the game. Lazy or disinterested players should be encouraged and poked.
Focus on the players, not your plot. Follow their interest. Reincorporate things that are important to them. Give them strong hooks so that play isn't directionless, by all means, but don't drag them along against their enthusiastic consent.
Ask questions and use the answers to colour your own narration. This can be little flavourful things to make the game richer, like, 'How does the Warp feel to you?'; 'What's your opinion on the Astartes?' or it can be more dramatic stuff like 'What was the first xeno you let live? Why?' or 'Which member of the group do you trust the most? Why?'. This is especially useful during character creation, to establish some background, themes and motifs for the game, especially the shared history of the group (if any).
Get players to contribute. This can be done in a more structured way too, like, whenever you go to a new area, go round the table and each player chips in a colourful detail about it. And if PCs aren't present in a scene, offer the players NPC roles to play instead. These techniques add richness to the game and keeps everyone involved.
Be practical
Take breaks, at least once every one and a half to two hours.
Assuming it isn't you, if someone is an expert on the rules, then let them be the expert on the rules. It's one less thing for you to worry about. If no one is an expert, then don't worry about it. Use the rules you can to serve the game. As soon as rules referencing and quibbling starts to bog you down and suck the energy from the game, then they're not helping you.
Make sure food and drink is available, if that's your responsibility too.
Good luck!