My experience was, that I became aware that God was calling me to preach because God was giving me something to say. As I read the bible, or listened to others preaching, or talking, something would also drop into my own heart.
As I shared the thoughts I received, I found that people were blessed and encouraged. This is vital when you are a preacher.
Study, bible school, may be the next step. But my thought would be that you have to be aware, or know that a gift is there in the first place. No amount of study can replace a gift.
The bible exhorts those who minister "to wait on their ministry" That means that with bible open and prayerful spirit as you read, you wait to hear from the Lord. This can take hours. But it is the only way to hear from God, and receive something for the people.
Just my own thoughts........nothing written in stone.