Meet up with Galmar and listen to his speach. Charge to the barricades and kill the enemies there and eventually the barricade should go down. Move up the road a little ways and try to get up on the broken wall section to the right. Get up there and then go up to above the draw bridge and release it for the Stormcloaks.
Make your way through town, up to the palace and fight the Jarl. Take him down and accept his surrender. Listen for a while and then head back to Urlfric.