So, you want to free the land of the Imperials, huh? Can’t say I blame you, they DID try to cut off your head after all. Well, chances are that if you’ve talked to any Stormcloak soldiers or Stormcloak supporters you already have the miscellaneous quest "Join the Stormcloak rebellion". But what you want to do if you have a mind to knock around some Imperial skulls is to travel to the city of Windhelm. Head to the back of the city and enter the "Palace of the Kings". Here you can find the leader of the Stormcloak rebellion, Jarl Ulfric Stormcloak. If you ask him if you can join, he well tell you to talk to his second in command, Galmar Stone-Fist.
Talk to Galmar Stone-Fist, who will ask why you want to join them if you’re not a Nord. Answer as you will and he will give you a test before he lets you join. He will want you to go to Serpentstone Isle to kill an Ice Wraith. This ends the beginning miscellaneous quest and starts a MAIN quest entitled "Joining the Stormcloaks".