I made a post covering some basic stuff recently here:
http://www.neogaf.com/forum/showthread.php?p=99046664&highlight=#post99046664
For what its worth, you're making the hardest transition possible - Ghosts is
incredibly unfriendly for rushing with an smg/shotgun as a new player.
Note that it is still very much possible, a friend and I both run shotguns and smgs regularly on several maps now, and do well with them, but we were getting destroyed in the first month or so when we tried them.
These maps are significantly more open and have much less obvious flow than the BO2 maps. Its there, and they do have it, but it'll take you a bit to learn where people like to move and camp.
Try to fall back to a midrange zoning style of play with an AR instead of an aggressive pushing style - the spawns
will fuck you until you realize how they operate (in Dom they're anchored by a) flags b) teammates and c) enemy line of sight NOT enemy proximity - in other modes they're similar, just no flags).
I'd also recommend sticking to Dom or Blitz until you learn the maps, TDM will make you hate both the spawns and the campers until you know the maps better.
For streaks, if you use Assault, Hardline with 5/7/9+ is the easiest, use IMS and Sentry. Dog is mostly useful for revenge buzzkills, it's bad at actually propelling your streak. Use Trinity instead of Sentry if you're only aiming for a heli at 9, Sentry is better for the 9+ streaks like Helo Pilot, it can do a lot of damage if placed well.
Support is extremely powerful in this. Vests, MAAWS, Squadmate, Ground Jammer and Oracle are all really strong.
Specialist is... specialist. Fun if you're kem hunting or want to experiment with builds using multiple expensive perks.
But yeah, brass tacks, take an AR with a silencer and second attachment of your choice, Stalker, Focus, Dead Silence, Amplify, Blast Shield or Sitrep. If you have trouble controlling your sprinting, swap out Amplify or BS/Sitrep for Ready Up and Takedown.
For the spawns, watch this excellent series of videos, they carefully explain exactly how the spawns operate on every map in Dom, they'll accelerate your learning tremendously:
Intro:
http://www.youtube.com/watch?v=fOEmtU9tgk4
Whiteout:
http://www.youtube.com/watch?v=iIQ64MuWG_c
He has one for all the mp maps, but just learning the basics and having a rough idea of the spawn locations is plenty to get started.