You will need to report to your commanding officer. They will send you off to join the forces massing to take this fort.
Join the Men attack the fort
The group of soldiers can be found at the base of a ridge nearby. This is to the North of the Fort. They are just outside the archer's range.
Take over the fort by defeating the enemy
When it comes time to take over the fort, you have a number of soldiers to deal with. They will be on the walkways above the entrance. Take your time to clear out the soldiers that are on the ramparts.
With the outside forces dealt with, head on inside. There are a number more archers on the spires of the fort inside. This is in addition to the numerous soldiers spread throughout the area. Make your way up the stairs and press toward the back of the fort.
Work your way steadily back through the fort. Toward the end of clearing out the fort, more Soldiers will be clumping together. Just be more careful in these times.
Report to Legate Rikke
Report to Ulfric