All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.
You are obliged to turn to Allah in a sincere repentance. In addition, you should feel deep sorrow for what has happened. Moreover, you should keep asking forgiveness of Allah and performing as many good deeds as you can. If you fear Allah and be honest with Him surely Allah will forgive you. Allah Says about the greatest sins such as Shirk, killing innocent person and Zina (what means): {... And whoever should do that will meet a penalty. Multiplied for him is the punishment on the Day of Resurrection, and he will abide therein humiliated - Except for those who repent, believe and do righteous work. For them Allah will replace their evil deeds with good. And ever is Allah Forgiving and Merciful. And he who repents and does righteousness does indeed turn to Allah with [accepted] repentance.} [Quran 25: 68-71]
You should also take lessons from what happened. Hence, you should not take easy the commands of Allah and should not be close to the prohibitions of Allah. Adhere to the rulings of the Sharee’ah; it is purer and safer for you. It protects you from scandals in this life and in the Hereafter as well. You should also conceal your sin and do not inform anyone, even the future husband about what happened; this is the teaching of the Prophet Muhammad . It was reported in Musnad Ahmad that a woman came to the Prophet after getting pregnant from Zina. The Prophet instructed her not to inform anyone else. Abu Bakr and ‘Umar also did the same as narrated in Muwatta’ that a man came to Abu Bakr and said to him: “I have committed adultery.” Abu Bakr said to him, Have you mentioned this to anyone else? He said: “No.” Abu Bakr said to him: “Then cover it up with the Veil of Allah as He accepts repentance from his slaves. He was still unsettled, so he went to ‘Umar . He told him the same as he had said to Abu Bakr, and ‘Umar told him the same as Abu Bakr had said to him. The Prophet also said: “Avoid these filthy things [sins] which Allah has forbidden, but if anyone commits any of them, then let him conceal that which Allah has concealed, and let him repent, for whoever discloses to us what he has done, we will carry out the punishment prescribed in the Book of Allah the Al-Mighty on him.” [Al-Haakim and Al-Bayhaqi from ‘Abdullah Ibn ‘Umar]
Moreover, you have to wait one menstrual period before you marry the person who is going to marry you. This is only to see whether or not this illegal sex caused pregnancy. If pregnancy has occurred then the waiting period extents till giving birth. This ruling is based on the Hadeeth of the Prophet where he said in regard to the slave woman: “It is not lawful for a man to have intercourse with a captive woman till she is free from a menstrual course; and it is not lawful for a man to have intercourse with a pregnant woman until she gives birth.” [Abu Daawood] This Hadeeth indicates that a man should not have sex with a woman who is not pregnant by him until she has given birth.
Allah knows best.