Report to General Tullius in Solitude for a promotion and a reward for taking Whiterun. He will direct the Dragonborn to Legate Rikke to receive the next objective in taking Skyrim back from the Stormcloaks. After speaking with him, report to Legate Rikke at an Imperial camp. She will give objectives for each hold that must be taken, after which the Dragonborn must report back to Tullius, then return to Rikke at a new camp for the next objective. The first of these is A False Front.
360 PS3 Legate Rikke will not give the quest "Rescue From Fort Kastav."
360(Fix) PS3(Fix) Patch 1.6-Go to the nearby cooking spit and activate it, this should trigger the "Reporting for Duty" dialog option to appear. Works for PS3 and Xbox 360 but NOT on PC.
360(Fix) PS3(Fix) Reloading a previous save or saving and loading may also fix this problem.
PS3(Fix) If Markarth is given to Ulfric Stormcloak during "Season Unending," clearing Fort Sungard from the Stormcloak army and then fast traveling to Rikke's camp will give the "Reporting for duty" dialogue. Going through the rest of her dialogue options might be necessary as well.
If the Reach was given to the Stormcloaks during "Season Unending," then it will be the Reach instead of the Rift. When talking to Legate Rikke, the "Reporting for Duty" dialogue option may not appear. This may be able to be fixed by using all three words of Unrelenting Force on Legate Rikke.
During the quest to retake Fort Sungard, the Imperials and Stormcloaks may not appear, leaving the fort empty.