What are the pieces of advice and guidance during the period of preparing oneself for marriage while maintaining admiration for a girl in the heart without disclosing it to her, due to young age?

If the girl is religious and of good character, there is no impediment to marrying her, and it is preferable to formalize the marriage contract. Allah has promised wealth to those who marry for the sake of chastity, and the Prophet, peace and blessings be upon him, mentioned that among those whom Allah is obligated to help are those who marry seeking chastity. Young age is not an impediment to marriage if the young man has the ability and desire. One must avoid being alone with the girl or engaging excessively in conversation and looking at her before the marriage contract. As for merely having one's heart attached to her without committing any forbidden acts, there is no harm in that.