Self explanatory. As soon as you reach Level 10, the trophy is yours.

If you are new to the series or FPS games in general, you'll be happy to know there's a game mode entitled 'Boot Camp'. This mode is found under the Combat Training playlist and is only available for players under Level 12. This is where you can get to grips with the game and learn the basics with other newcomers. After you reach Level 12, the game mode is locked permanently. 

Black Ops II features many different game modes, most of the old ones like Team Deathmatch and Domination return alongside a few new modes added like Hardpoint and Multi-Team. Most of the maps are small to medium sized and some of them, like Express and Slums, are designed to have 3 or 4 different "lanes" you can attack from while the larger maps, like Carrier and Aftermath tend to have wide open spaces. 

The Create-a-Class system has been revamped with the Pick 10 system. Everything you add to your Loadout, be it an attachment, a Perk or a Lethal will take up a single point with 10 being the maximum. This system is designed to be flexible and caters to your playstyle. If you have a speed reliant playstyle, you can forego any equipment to bolster your speed capabilities. Have more of a focus on equipment? You can stock up on Lethal and Tactical supplies in favour of Perks or attachments.

The main difference to the Multiplayer is the removal of Killstreaks in favour of Scorestreaks. In previous games, only getting kills would grant you Killstreak Rewards. The problem with this is that not many people would interact with the objectives in objective-based game modes, as players would favour acquiring Killstreaks over objectives. With the Scorestreak system, players obtain Rewards based on their score. Score can be obtained by doing pretty much anything, from killing enemies to destroying aircraft to capturing objectives with the most points being offered for capturing objectives. For example, a UAV used to require 3 kills but now it requires 425 score. A kill awards you with 100 score but capturing a flag in Domination or securing a Hardpoint grants you 200 score, twice as much as a kill, therefore incentivising objective interaction over kills.

As the objective oriented game modes are generally played by more experienced players and tend to be more Scorestreak dominant, for your first few levels, i recommend playing Team Deathmatch or Free-For-All. Once you're confident in your gunskill, try tackling the objective game modes and see which one you play best in.