You will get the knife from the chef. He is near where you can collect avacados and an egg from the cliff. Give him an egg when you approach him and then talk or offer help, you need to keep giving a gift in order to receive relationship points with each person you talk to. It took me a few times to figure this out, I was wondering why he was so mad when I talked with him. Anyhow, you will need to speak with Ms. Weeds in order to eventually trade her for some lime seeds (which you then grow/water to get the limes for the chef) To get the sardine I believe he wants, you need to spearhead for that fish on the beach. And keep in mind that different fish can show up depending on if it is a certain time of day.
* Also try spearheading for fish in the river & the fresh water lake near Ms. Weeds. As you will need different kinds to make items.
Just remember that everytime you "offer help" to someone and complete the task, you will then receive something you need from that person, or you will have a new option to trade for something you couldn't get before.