First of all, you should be a Catholic in good standing, that is to say, in communion with the Church. This means that if married, your marriage should be a valid one, blessed by the Church. If you are divorced, but, not remarried and not in any kind of relationship, then you could also participate. You would also have to be at least, in most cases, 18 years of age or older.
The pastor will talk to you about it. In our case, our names are submitted to the bishop for his approval. Then, if everything is in order, then you will undergo training.