The first sign of John's mission and purpose is when he is very young, and takes a piece of metal and is able to hold it without dying. He writes of this experience that because of it, he knew he was destined to become a priest also. That was his first sign--it is symbolic because supposedly, only priests were able to hold the metal without getting sick. So, his holding it symbolized his "career" path in life. The next sign of his journey comes as he tells his father of his dreams, after he asks to go on his journey. The dreams reveal the city of the gods, with gods walking in them. His father agrees that this is a sign that he must go on his "spirit walk," and John takes the dreams as a sign that he has been granted special permission to go to those forbidden places. It isn't allowed, but since John dreamt of them, he feels it is his special mission, so, he goes.
To decide which direction to go, he follows signs given to him--an eagle and three deer, one of which is a white fawn, that are going east. The eagle came first, but to ensure it wasn't a "bad" sign he waits for the deer. The white fawn, probably a great symbol because of its rarity, confirmed his direction. In ancient societies, they were very suspicious, and looked to signs in nature to determine their course. The rest of his journey is motivated mostly by his burning desire for knowledge; he knows he shouldn't go, but does anyway, with unease in his heart, and full expectations of being punished or dying for disobeying. He figures not being killed by the Forest People is one sign of his destiny, but rather stretches it to fit his own desire to learn. So his journey starts off sanctified by signs, but, he takes his own initiative to go the rest of the way.
I hope that those thoughts helped a bit; good luck!