Well, Im finally going to give Dm-ing a try. I have exp with DnD, a tiny bit of dark heresy and this will be out first time playing deathwatch. Our group really likes fighting, Ive mapped out a space hulk scenario. Our group consists of a DA devastator, BA assault, SW librarian, BT apothecary, and UM techmarine. Any tips or ideas?
Space Marines may require some special handling compared to what you're used to as they're obviously all superhuman fighting machines, so it's easy to go into scenarios that have only combat in them. Make sure they have the opportunity to grow into full characters rather than just armour-shaped cardboard cutouts.
I'd suggest tossing them at least occasionally something the various Chapters might normally approach differently, such as various regular humans - e.g. they're all likely to treat someone like an Inquisitor, Adeptus Ministorum Cardinal or Sanctioned Psyker in various ways. You've already got a source of potential initial disagreement between having both a Black Templar and a librarian in the same group, but even in the midst of a series of battles you can push a button with little things like the vaguest hint of Fallen activity at a bad time.
Ask not the Eldar a question, for they will give you three answers; all of which are true and horrifying to know.