I watched tarot card videos on YouTube before a test.i watched a few of them .I believed in them that night but I realized it was wrong and in the morning I stopped believing them. I know it is a sin but is it a kufr . If so how should I repent
Allah SWT is the All Knowing and only He knows about the Unseen. Believing in these is a form of kufr.
Allah SWT is the Most Forgiving and the Most Merciful. If a person repents with sincereity and genuine remorse and resolves to not commit the sin again, then Insha Allah, Allah SWT will forgive him.
“Say to those who have disbelieved, if they cease (from disbelief), their past will be forgiven” [al-Anfaal 8:38]
Here are more references: Repentance
Allah says in the Holy Quran: “And with Him [i.e., Allah] are the keys of the unseen; none knows them except Him.” [Qur’an 6:59]
As well as (what means): “Say (O Muhammad): ‘None in the heavens and earth knows the unseen except Allah…’” [Qur’an: 27:65]
Allah’s Messenger, (pbub), was told by Allah to say (what means): “…If I knew the unseen, I could have acquired much wealth, and no harm would have touched me. I am not except a warner and a bringer of glad tidings to a people who believe.” [Qur’an 7:188]
And the prophet (pbuh) “Whoever approaches an oracle or fortune-teller and believes in what he says, has disbelieved in what was revealed to Muhammad.” [Ahmad & Abu Daawood]
It is important that you ask Allah for forgiveness and realize that only Allah knows the unseen, which includes that which will take place in the future. Your best source of help is in Allah so call upon Him in your time of need. And know that Allah loves to forgive His erring servants.