As you level up, you will be allowed to spend attribute points into the various statistics to improve them. Consider putting all your points into the energy meter to allow yourself to play more frequently. This will allow you to progress faster through the early part of the game, and begin gathering agents and money. Your energy will be refilled completely each time you level. With careful timing, you can level up while your energy meter is low and extend your game time. Play player vs. player missions earn some experience. Although it will not be as much as regular missions, it is still helpful.