Advocacy jobs are as varied as the legal system itself. In some cases, you may become involved as an advocate on a purely volunteer basis. For example, if you volunteer at a homeless shelter to help clients find lodging, your volunteer status may be because the organization doesn't have the money to pay a formal advocate. In such cases, experience requirements are usually much lower, and the job itself can even be learned through on-the-job experience alone.
If you are interested in becoming an advocate, it may be helpful to volunteer first in the area that you are most interested in. From there, volunteer advocacy jobs can often become paid jobs. Some advocates, too, eventually find a niche they are particularly interested in, and go to school specifically to become lawyers in that particular area.