In Tenel, go to the blacksmith's shop and talk to him. He will give you a Fruit Knife. Go to Madril and enter the shop. Talk to the kid at the counter and give him the knife. He will give you 300 Sukel.

When you get a new sword, sell your other weaker swords.

When you get though the Aquatic Ruins, you can keep returning to fight the earns to get more Sukel and experience.