You cannot control your feelings. You can control the actions which result from those feelings.
If you fall in love with someone, then that is not a sin. If you begin to act on that love without getting married, that is a sin.
When you fall in love with someone it is best to marry them. The following hadith is evidence that you can fall in love before marriage.
The Prophet (peace and blessings of Allaah be upon him) said: “We do not think that there is anything better for
those who love one another than marriage.” (Narrated by Ibn Maajah, 1847; classed as saheeh by al-Busayri and by Shaykh al-Albaani in
al-Silsilah al-Saheehah, 624)
For a complete and detailed explanation
please do read this.