"Imperio" a young Tom Riddle said. It was just too easy for him.
Myrtle Warren was crying her heart out inside a stall in the first-floor girls' bathroom at Hogwarts. If only Olive Hornsby wasn't an utter prat, she wouldn't be crying and fully unaware of her surroundings.
Tom had just come out of the Chamber of Secrets after talking with his inherited basilisk. His basilisk wanted a warm body for his reward or else he would stop listening to his commands. Luckily for him Myrtle was there and pliant of mind.
He ordered her to come to him thinking, that he could lead her to the chamber to die. But the basilisk had to be a busybody and look at the girl. It was an accident but he forced her to look head on. Thus, his body reacted violently to his crime.
He threw up on his diary - a white viscose material soaking it through. That was his soul making his first Horcrux. However, he couldn't savour his first step into madness -or greatness, depending on who you asked -as Hornsby was coming to taunt Warren. As he left, Warren's body was becoming too cold for his basilisk but it was better to cut his losses and blame his success on the perfect scapegoat, Hagrid, the half-breed.
The first cut of his soul was the deepest.