Mass Effect 3
68 Achievements
Enlisted
Start a character in multiplayer or customize a character in single-player.
How to unlock the Enlisted achievement in Mass Effect 3 - Definitive Guide
Also if you wanna jump into Multiplayer first you can create a character from the 6 choices listed.
*Edit: KP ScuzzyBunny
If you use the default character in single-player, you will not get this achievement. If you use a customized one, you will at the start of the mission (when you can actually walk around). If you start up a multiplayer, you will be given the option to customize a character as well. This is an easy achievement and very well may be the first. If you wish to use default, then you can also start a random game just for the achievement then erase it.
There are two methods to obtaining this trophy. Both are listed below.
Single player: Create a new character and choose to customize Shepard's appearance over using the default choice. Once you have customized your character’s appearance and start up a new game, this will unlock.
Multiplayer: Start up the multiplayer mode, choose a class, and enter your character’s name. Once you are at the next screen to enter a match, this will unlock.