Head over to Avina. When Shepard gets near Avina some husk will drop down and attack. Kill them.
After the fight, talk to Avina.
After the conversation, go to the elevator. Use it. A cut scene will begin of the fleet defending the Citadel.
After the cut scene the party will be out of the elevator. Follow the path. When the path goes down an elevator will go by and drop some Geth Troopers off. Kill them as Shepard follows the path.
After the path goes back up, there will be some Geth Destroyers and a Krogan Warlord.
After the fight will be an opening to the right. Follow the path up the ramps. There will be some Geth on the ramps and after it to the left. Kill them.
After combat follow the path to the left. Keep going straight or to the left. When Shepard reaches the bridge some Geth Destroyers will attack. Kill them. Cross bridge.
After the bridge will be another open area. A Geth drop ship will land, sending out Geth Troopers. Kill them. If Shepard activates the turrets in the area, they will help destroy the drop ship.
After the battle, go south. At the far end will be an opening. Enter it and follow walkway.
As Shepard rounds the corner some Geth and Krogans attack. Wipe them out. Follow the walkway. There will be a tough battle ahead in the next area. There will be an assortment of Geth Troopers. As an added bonus there are also Geth Heavy Turrets. There are two choices here. Fight while running straight down the middle, or Shepard can take a path on the left side of the area and just fight Krogans. The left path is easier. After the fighting is done, Shepard needs to get to the far end and be on the left path.
Near the end of the path will be a door to the council chambers. Enter it.