This will be your first assignment of the civil war. You can join the Stormcloaks by talking to Galmar Stone-Fist (Windhelm) or enforce the imperial ranks by talking to Legate Rikke (Solitude). The two will give you a misc. quest to complete to prove your worth; find the Serpent Stone and kill an Ice Wraith for the Stormcloaks or clear out Fort Hraggstad for the Empire. You will then have to take the oath for either side. This ends the preliminary quest, starts the associated faction objectives and gets you the trophy.