In 1 Nephi chapter 1 we learn a number of interesting things. First that Nephi is a young man who has already had spiritual experiences. He comments on his great knowledege of "the goodness and mysteries of God". This means that he has diligently sought to know God and to seek wisdom from Him. We also learn a little about Lehi. My feeling is that he was not yet considered a prophet, but he heard the other prophet talk about the coming destruction of Jerusalem and had to know for himself (what a great idea). As he prayed the Lord revealed may great and wonderful things, including about Christ and his 12 apostles going forth. I think after this experience he was compelled to go out and preach and prophesy.