Praise be to Allah.
You have to ask Allaah for forgiveness and repent to Him for what you have done, and you have to resolve never to go back to that. The fact that you touched this woman does not mean that you are obliged to marry her; there is nothing in Islam that compels a sane adult to marry any woman whom he does not want to marry. No marriage is valid unless it meets the necessary conditions, one of which is the consent of the husband who is accountable.
You do not have to tell your parents or the girl you are going to marry about what happened; rather you are obliged to conceal what you have done, and to repent, and this is between you and your Lord. The Prophet (peace and blessings of Allaah be upon him) said: “Avoid this filth that Allaah has forbidden. Whoever does any such thing, then let him conceal it with the concealment of Allaah.” (Narrated by al-Bayhaqi; classed as saheeh by al-Albaani in al-Silsilah al-Saheehah, no. 663).
“Filth” means evil actions and bad speech that Allaah has forbidden. Subul al-Salaam, 3/31
And Allaah knows best.