Thank you for your question.
Since you are now an adult, the way to prepare to make your First Holy Communion is to drop around to see the priest after Mass any Sunday and ask him to help you. You mentioned that you have already made your First Confession, so any priest would be able to help you to prepare to receive Our Lord in Holy Communion. Do not leave it any longer. Now that you have raised the question, follow it up.
May God bless you for your interest in receiving Him.
Fr. Jordan O.P.