Get a clue.

"Thanks, I could really use the help", you say. "Can you tell me the clue?"

"I tell you", Cedric replies, "but I can do even better. I can show you."

"Thanks", you say, meaning it from the bottom of your heart. "Lead away."

A grin lights up Cedric's face. You run up to your room, and bring the Golden Egg down to the Quidditch pitch. Cedric leads you to the large shed where Quidditch equipment is kept. Inside, among other things, is a large pool, where rogue bludgers are kept contained. Cedric leads you over to the pool, and after checking that it's empty, uncovers it.

"All you need to do is listen to the egg underwater", he explains. "There's a riddle. I don't know what it means; you'll have to work that out for yourself. But at least, you know how to get it."