Shamanism is one of those things you can read about in every book in the world and still not really grasp. This is because words live in one world and the roots and reach of the shaman lie in others. Access to these other worlds is not defined by intellectual understanding, but by direct experience - experiences that we call initiation. An initiation is a doorway opening into these other worlds. It might take many forms in this world, but if it is an effective initiation, it will have the impact of opening your awareness to the worlds of the shaman.
It could be something as simple as a life threatening illness that shoves you out of your body and into an "hallucinatory" state in which you encounter beings or information that change your life. I could be something more specific, like a ceremony in which your physical body is ritually dismembered and your organs replaced with glowing gems.
Initiations generally take one of two paths. Either they occur naturally - illness, car accident, lightning strike - or they are planned and orchestrated by those who have already encountered these doorways and know how to acquaint you with them in a safe and relatively non-traumatic way. If you have the choice, I encourage you to seek the later. They are much easier. Still terrifying and overwhelming, but much easier than the initiations that spirits provide you with if they feel you are not responding to their calls.
Those initiations that are encountered haphazardly in life seem to be sent by spirits that have decided they have a use for you. It would help if we lived in a culture that recognized such encounters for what they are and responded appropriately. Instead, what often happens is that the spirits get frustrated when we don't respond and so they knock louder. Eventually we either wake up or we don't survive.
I don't mean to say that every car wreck, epileptic seizure or coma is a call from the spirits. This is another way in which words to not begin to express the nature of what it. There is a unique quality to an experience that is an attempt by spirit to wake you up. You may experience this quality in the disorientation of your consciousness and the sudden awareness of things that you didn't know before, but suddenly realize to be true. It can all be quite confusing - and there is little difference between someone called to be a shaman and a madman.
Of course, as indicated by another meaning of the word, initiation is only the beginning.