While many things have been simplified in the Mass Effect Legendary Edition, some important secrets are still being missed. It's hard to say any of these secrets are more important than class specialization. Mass Effect 2 and Mass Effect 3 make achieving this upgrade obvious from the very start of character creation, but not the first game.
Many players are going through their games and completely missing out on this entirely, which is a tragedy because the specialization for each class is awesome. Unlike most other video games, this isn't unlocked with experience or talent trees, but with a specific and strangely optional mission. Don't let Shepard suffer because of a mistake, make getting this specialization a priority.
Where To Go When Getting The Normandy
This can't be unlocked until getting the Normandy because it's in a different cluster than the Citadel. So hurry and finish things up with the Council before venturing into space.
You'll want to head to Sol in the Local Cluster. This should be Shepard's first destination after leaving the Citadel. Though it's tempting to do some of the other intriguing side missions or pick up Liara T'Soni and complete the squad, it's more important to have a character that is leveled properly. Some players might save the optional missions like this one for the end, but they'll have run out of experience points and levels available. Basically, the new skills gained can't be maxed out. And they will come in handy for the final fights.
Head over to the Milky Way system and visit the Local cluster. This layout should all look very familiar; this is where Earth is! Where Shepard needs to go isn't far from home. After entering the system, Admiral Hackett will speak to Shepard about a Rogue VI. The only location that can be interacted with is Luna, Earth's moon. Set a course for there and hop down.
Defeating The Rogue VI
Upon landing on Luna, note that the facility will be marked on the map. Drive over with the Mako and take out the turrets (there will be two at each of the three facilities).
Each of these bunkers needs to be explored. The map is too large to tell north, south, east, and west, so a pro tip is to park the Mako in front of one facility, then investigate a different bunker first. Upon leaving, head to the bunker that doesn't have the Mako parked in front. Then finish up with the bunker with the Mako last. It helps to keep from getting lost.
Inside, there will be loads of floating drones that the VI is using to protect itself. They are very deadly but they can be easily grouped together by retreating into the hall that Shepard entered from. This will make for a line-of-sight problem and Shepard can shoot around the corner to dispatch them. Also, using anti-synthetic mods on weapons will do extra damage here since all of the opponents are mechanical in nature.
Each tunnel will add a complication to the next one. The first will create shields and the next will add toxic gas. For the gas, equip medical mods in Shepard's armor to counteract the health decay.
Shutting down the VI will give Shepard a choice of specialty to unlock. Congratulations!