Okay, three questions:
1. How do warhammer 40k miniatures work? Are they like little pieces you combine or premade models or molded parts you glue together...?
2. Warhound Titan or 3 Deadbanes?
3. All Space Marine army, Mixed army with various Imperial factions and Space Marines, or all Imperial Guardsmen?
1. They come in pre-molded sets like these:

this is just from a tyranid gaunt set, actual pieces will vary obviously. You precisely cut them out with pliers, glue them together, then paint (I'm not sure if the game itself requires painting but tbh it's just why wouldn't you)
2. no idea what a deadbane is, but AFAIK titans are for the "epic" scale games where regular infantry are just around 6 mm tall as opposed to around 28 mm, I'm probably wrong and someone like Qwepir/Tingals/Comr4de would be better able to answer
3. For the actual game I don't think you can really mix factions. I don't own a copy of a rulebook yet, but I think it's a safe bet that you can't really mix.