First off, you need to hook up with some folks and play with them. This game is infinitely more enjoyable with a team. You will have more success and learn from them.
I recommend playing Squads for a while. Play around with setups, learn the maps, and experiment with the streaks a bit. All while earning XP. It will help you get comfortable with controls and how to move about the map.
I'd say learn the maps and the general spawn system. It's still flawed as is, but it'll give you an idea of where the enemy team should be spawning if your dummy teammates aren't pushing aggressively. Also experiment with the perks to find what's best for your play style and set up. I use turtle beaches, so amplify is on all of my classes and occasionally dead silence. If you're getting beat in all firefights, put on ready up and quick draw to help get the jump on enemies.
As for weapons, go with the AK with silencer & extended mags. Or Remington with RED & silencer or extended. Start there and play with your set ups and you'll get better. The kids that play this game for the most part aren't as good as past games due to more reliance on personal skill and less on air streaks which decimate the team for you.
Play TDM. Slow you game down until you can pick up on patterns and figure out player tendencies. Learn gun strengths and figure out which weapon suits you the most. Situational awareness is a key in this game because the maps put you at risk of being flanked all the time. Learn how to best keep the enemy in front of you.
Don't die. Stay alive and the kills will find a way to fall in your lap. Black Ops II was "fast paced" and far more chaotic than Ghosts. However, in Ghosts I camp much harder but I still average the same number of kills per match even though I'm playing TDM instead of DOM. Why is that? I'm patient and always making the most out of each encounter with the enemy.