He had first heard about the Chamber of Secrets five years before and had spent that time working out there it could be located and how to enter it. When he finally does open it, he uses Parseltongue to control and release the Basilisk within. A Hogwarts student, Myrtle Warren, is killed by the monster shortly thereafter in June.
Tom Riddle explained to Harry that it took him five years to work out how to enter the Chamber:
... but I admit, even I was surprised how well the plan worked. I thought someone must realize that Hagrid couldn't possibly be the Heir of Slytherin. It had taken me five whole years to find out everything I could about the Chamber of Secrets and discover the secret entrance ... as though Hagrid had the brains, or the power! (CS17)
Since he opened the Chamber in 1943, he must have first come up with the idea in 1938, when he was in his second year.
Myrtle was killed in June, but we don't know how much time had passed between Riddle's first foray into the Chamber and that death. It's likely that the events were not concurrent, as Riddle would likely have entered the Chamber several times to explore and to discover and learn to control the Basilisk before the incident with Myrtle. For the purposes of this timeline we'll assign a date of May 1943 for the initial opening to allow for a few weeks between that and the murder. However the dates could be closer together (both in June) or the initial opening of the Chamber could be even earlier, although this seems less likely.