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.
Zina, without a doubt, is a major sin. Allah Says (what means): {And do not approach unlawful sexual intercourse. Indeed, it is ever an immorality and is evil as a way.} [Quran 17:32]
Thus, one who commits such a sin must repent faithfully. A sincere repentance must be accompanied with regret, seeking forgiveness from Allah and genuine resolution to stop doing such a bad deed again.
In the same vein, one has to do more and more good deeds such as reciting the Quran, remembering Allah, charity, etc.
Allah Says (what means): {…and do not commit unlawful sexual intercourse. 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.} [Quran 25:68-70]
Also, we advise you to follow the Prophetic advice.
‘Abdullah Ibn Mas’ood narrated that the Prophet said: “O young people! Whoever can marry should marry, as it helps to lower one’s gaze, and protects one’s private parts, and whoever is not able to marry should fast, as this helps him to protect his private parts (from committing illegal sexual intercourse).” [Al-Bukhari and Muslim]
So, if you can afford to marry, you have to marry. Marriage is a good deed and you will be rewarded for it instead of committing what is forbidden for which you will be punished.
But, if you cannot marry, we advise you to spend your time in doing good actions, taking good friends, visiting righteous people and so on.
In addition, we remind you of the Prophetic Hadeeth, “He who abstains from doing bad deeds and indecent things, Allah will keep him safe (from such actions).” [Al-Bukhari]
Allah Says (what means): {But let them who find not [the means for] marriage abstain [from sexual relations] until Allah enriches them from His bounty.} [Quran 24:33]
As for drugs and narcotics, it is strictly forbidden to take any kind of drugs or narcotics because they are harmful and malignant.
Allah Says (what means): {who enjoins upon them what is right and forbids them what is wrong and makes lawful for them the good things and prohibits for them the evil and relieves them of their burden and the shackles which were upon them.} [Quran 7:157]
Besides, the Prophet said: “Every intoxicant is Khamr, and every intoxicant is prohibited.” [Muslim]
He also said: “If a large quantity of any beverage intoxicates, then a small amount of it is prohibited.” [Ahmad]
As for your third question, please refer to Fataawa 82320 and 81545.
Allah knows best.