Prophecy is saying what God wants said. If He calls you to it, you will start doing it. Most of what is called "training" is people's opinion. There is some good advice in I Cor. 14 about how to use prophecy in a group. Belonging to such a group is a good way to find out if that is your call. Although, sometimes God calls people to prophecy independently, like the prophets in the OT. Bob Dylan once observed "come poets and writers who prophesy with your pen", and Simon and Garfunkel observed "the words of the prophets are written on the subway walls and tenement halls". Being a prophet is pretty much following God on an individual call, but doing it as part of a group to help keep them grounded in God's purposes.