There are many forms of Zina, but what we are addressing here is the highest form. This is adultery, fornication, having sexual intercourse without being married to the person.
Allah (SWT)'s order in the Quran to stay away from Zina.
"And those who invoke not any other god along with Allah, nor kill such life as Allah has forbidden, except for just cause, nor commit illegal sexual intercourse (zina) and whoever does this shall receive the punishment. The torment will be doubled to him on the Day of Resurrection, and he will abide therein in disgrace; except those who repent and believe and do righteous deeds, for those Allah will change their sins into good deeds, and Allah is Oft Forgiving, Most Merciful." (al-Furqaan #25, ayat #68-70)
"And come not near to unlawful sexual intercourse. Verily, it is a faahishah (a great sin) and an evil way." (Sura Al-Israa # 17 ayah # 32)
Imaam al-Qurtubi (may Allah have mercy on him) said: The ulama said that the phrase And come not near to unlawful sexual intercourse is more eloquent than merely saying Do not commit zinaa, because the meaning is, Do not even come close to zinaa. This means not doing any deed that may get close to zinaa or lead to it, such as being alone with a member of the opposite sex, touching, looking, going to evil places, speaking in a haraam manner to a woman to whom one is not related, thinking about and planning immoral acts, and so on.
"Tell the believing men to lower their gaze (from looking at forbidden things), and protect their private parts (from Zina illegal sexual acts, etc.). That is purer for them. Verily, Allah is All-Aware of what they do.
"And tell the believing women to lower their gaze (from looking at forbidden things), and protect their private parts (from Zina illegal sexual acts, etc.) and not to show off their adornment except only that which is apparent (like palms of hands or one eye or both eyes for necessity to see the way, or outer dress like veil, gloves, head-cover, apron, etc.), and to draw their veils all over Juyubihinna (i.e. their bodies, faces, necks and bosoms, etc.) and not to reveal their adornment except to their husbands, their fathers, their husband's fathers, their sons, their husband's sons, their brothers or their brother's sons, or their sister's sons, or their (Muslim) women (i.e. their sisters in Islam), or the (female) slaves whom their right hands possess, or old male servants who lack vigour, or small children who have no sense of the shame of sex. And let them not stamp their feet so as to reveal what they hide of their adornment. And all of you beg Allah to forgive you all, O believers, that you may be successful." (Sura # 24, Ayat #30 and 31)
Hadith warning against not even coming close to Zina
"Some persons from Banu Hisham entered the house of Asma' daughter of Umays when AbuBakr also entered (and she was at that time his wife). He (AbuBakr) saw it and disapproved of it and he made a mention of that to Allah's Messenger (saaws) and said: I did not see but good only (in my wife). Thereupon Allah's Messenger (saaws) said: Verily Allah has made her immune from all this. Then Allah's Messenger (saaws) stood on the pulpit and said: After this day no man should enter the house of another person in his absence, but only when he is accompanied by one person or two persons." (Sahih Al-Bukhari Book 25, Hadith # 5403)
"Abd-Allaah ibn Masood (RAA) said: I asked the Messenger of Allah (SAW), Which sin is worst in the sight of Allah? He said, To make any rival to Allah, when He has created you. I asked, Then what? He said, To kill your child for fear that he will eat with you. I asked, Then what? He said, To commit zinaa with the wife of your neighbour." (Reported in Sahih Al-Bukhaari, Hadith #492 and In Shaih Muslim, Hadith #90).
"Rasulullah (SAW) explained: If one of you were to be stabbed in the head with a piece of iron it would be better for him than if he were to touch a woman whom it is not permissible for him to touch." (Reported by al-Tabaraani; see also Saheeh al-Jaami, 5045).
This refers to the punishment for touching, so how about worse deeds, such as embracing and kissing, and even worse kinds of illicit activity?
Rasulullah (SAW) said, "Whenever a man is alone with a woman the Devil makes a third." (Al-Tirmidhi 3118, Narrated Umar ibn al-Khattab, Tirmidhi transmitted it as authentic) note: So we should always try not to be alone with a woman who is not mahrum to us and not even go close to Zina.
Repentance from Zina
Ibn Abbaas (RAA) said: There is no major sin if one asks for forgiveness, and there is no minor sin if one persists in repeating it.
So if one wants to repent from having made the great sin of Zina he should remember these 4 things..
1) Do not despair, for Allah the Most Exalted and Glorified said in the Quran "Say: Oh my servants who have transgressed against their souls! Despair not of the mercy of Allah, for Allah forgives all sins; for He is oft-forgiving, most merciful." [Surah 39, Verse 53]
2) Let your repentance be truly from your heart, and stay away from all sources of temptations. Also, perform many good deeds, as good deeds abrogate the bad ones.
3) If you repent to Allah, you are no longer described as a fornicator (zaani). Therefore, you can marry a chaste man or woman.
4) The believer has high hope and aspiration for the best from Allah. He not only asks Allah for making his punishment easy in hellfire, but he also prays to Allah the Almighty to save him from Hell and award him with paradise for his repentance and good deeds.