After defeating Yakra, you should go back to Guardia Castle.
New Game +
If you're playing a New Game + , check the sparkle in the right-hand telepod to go directly to to the final boss. If you win, you get the ending "The Successor of Guardia".
Go to Guardia Castle
Now rest in Crono's house if you want to. You can also introduce Marle to Crono's mother. When you meet new characters later, you can introduce them to Crono's mother, too. Now take Marle back to Guardia castle.
A cut scene will take place. When asked, say that you started everything, and say that you weren't tempted by her fortune.