*This solution contains a minor spoiler*
The achievement is simply to complete one multiplayer match or two N7 missions.
To play multiplayer you will need an online pass and Xbox live gold. If you have both of those then head over to multiplayer and play a match. At the end the achievement will unlock.
Or you can complete two N7 missions in the single player. You will begin to receive N7 missions after completing the mission on Mars.