When Ron and Harry find the diary, a name is written on it : "T. M. Riddle". If I'm not mistaken, the quote below is the first time we hear about the name Riddle:
Harry saw at once that it was a diary, and the faded year on the cover told him it was fifty years old. He opened it eagerly. On the first page he could just make out the name “T. M. Riddle” in smudged ink.
(Chamber of Secrets, Chapter 13, The very secret diary)
Ron knows the name because he polished Riddle's shield many times during detention, but otherwise they don't seem to know it.
“I know that name... T. M. Riddle got an award for special services to the school fifty years ago.”
“How on earth d’you know that?” said Harry in amazement.
“Because Filch made me polish his shield about fifty times in detention” said Ron resentfully.
(Chamber of Secrets, Chapter 13, The very secret diary)
Yet the name rings a bell to Harry... why?
And while Harry was sure he had never heard the name T. M. Riddle before, it still seemed to mean something to him, almost as though Riddle was a friend he'd had when he was very small, and had half forgotten. But this was absurd. He'd never had friends before Hogwarts, Dudley had made sure of that.
(Chamber of Secrets, Chapter 13, The very secret diary)