Brigadier General is level 70 in the ranking system. As you earn more and more experience from killing and completing objectives, you will slowly grow in level. As you level, you will be rewarded with a new rank, new weapon modifiers, and achievements. Below is a list of the amount of experience needed to reach the ranks related to experience:
Sergeant (lvl 7) ----------------- 12,500 XP
Second Lieutenant (lvl 38) ---- 260,000 XP
Major (lvl 50) ------------------ 400,000 XP
Brigadier General (lvl 62) ----- 660,000 XP
Currently, the fastest way to level in this game would be to play Deathmatch or Team Deathmatch games, as they offer a high amount of experience per kill and it is very easy to get multiple kills every game. Alternatively, you can boost control point hacking with a friend, but you will need a full party of six and other people can easily stop your progress.