How does one join the stormcloaks?

By asking around in Whiterun. Usually you can just talk to a bartender in one of the inns and ask for rumors or information. This will give you the Miscellaneous Task "Join the Companions". You can talk to the leader of the Companions in Jorrvaskr, their base in Whiterun. Jorrvaskr is located in the small area in front of Dragonsreach. It is a large building to the east (in whiterun)