"Matej is my best friend whom I tell everything to." is correct grammatically. "Matej is my best friend who I tell everything to." is colloquial English.

The translation given ("Matej is my best friend, whom I tell everything.") is missing a necessary preposition: "to". Correct answers would be: "Matej is my best friend whom I tell everything to." OR "Matej is my best friend to whom I tell everything."