Wellstone Action is a well renowned learning center, known for their excellent organizer training program. 

Their Organizing Tools section is a fantastic resource and goes in-depth, providing you with everything you need to know to be a successful organizer. With this resource there is definitely a heavy focus on political organizing, but many of the key lessons can be applied to all different kinds of organizing, so if you're not a campaign do be turned off by all the talk of getting votes.