Is it true that we're not allowed to kiss somebody before getting married to her/him according to Islam? If it is, what is the punishment for that?
About kissing a foreign woman i found this Hadith:
Ibn Mas'ud (May Allah be pleased with him) reported: A man kissed a woman. So he came to the Messenger of Allah (ﷺ) and informed him about it. Then Allah revealed this Ayah: "And perform the Salat, between the two ends of the day and in some hours of the night. Verily, the good deeds efface the evil deeds (i.e., minor sins)." (11:114) The man asked the Messenger of Allah (ﷺ) whether this applies to him only. The Messenger of Allah (ﷺ) said, "It applies to all of my Ummah."
[Shaih al-Bukhari & Msulim]
and this Fatwa in Arabic.
And don't forget a sincere repentance and expiation with real regretting and turn the sins into good deeds like praying, fasting,..., doing good things,helping: Being a good muslim and not redoing what those sins ...is necessary!
And Allah knows best!