After you complete Priority: SurKesh, you are able to start the second Mass Effect 3 N7 Mission, named N7: Cerberus Attack.
If you dont have it in your journal, talk to Specialist Traynor, immediately after you arrive on Normandy.
When to start
After Priority: SurKesh
Cerberus has taken control of an old military installation on Tuchanka for unknown reasons. Land on Tuchanka and neutralize whatever operation Cerberus has in the area.
Krogan First Division War Asset
To begin the mission, open the Galaxy Map in the CIC. Go back to Milky Way, and travel to Krogan DMZ. Enter the system and travel to Tuchanka. Land on the planet and select your crew, and weapons.
After you land, go straight forward until you reach a dead end. To the right should be a Cerberus Trooper and in front of you a large crate. Near it, in a corner is some junk that can be salvaged.
Completely clear the area, and make sure now Cerberus unit lives when you finish. You will encounter Assault Troopers, Centurions, Combat Engineers, as well as Guardians and Nemesis.
Head towards the Control Console and before going up the ramp, check the right side for another junk pile. Now, go up the ramp and pick up the Old Grid Schematics on the first desk, next to two holograms.
You will need these to finish the Citadel mission: Improved Power Grid. Go behind the control console and stick to the right side to get a Med Kit. Advance and go down the ramp to find the Armax Arsenal Gauntlets and some ammo.
Go back and activate the console, but get ready because more Cerberus will appear.
The position you have, up in the tower, is perfect for taking out the Cerberus. Ask your party to push forward, by sending them down, while you cover them from above. Locate the Power Modules you have to activate, and move forward, killing the remaining Cerberus.
The Power Modules are protected by two latches. Shoot them and reactivate the modules by pressing A.
Return to the console, but be careful because the Cerberus have the high ground now. Take them out, and activate the Control Console to finish the mission.
Rewards: +5 Reputation, 10,000 Credits and the Krogan First Division War Asset.
Next Mass Effect 3 N7 mission is Cerberus Abductions.