Congratulations! This is the day you will make your first weapon. The easiest to make, of course. With this weapon you can catch fish and kill animals*.
It's called "Spear". You can make many kinds of spears. First of all you'll need:
Stick
(You can find a lot in the mountain area. For the map, look here: http://www.gamefaqs.com/ds/935188-lost-in-blue-2/faqs/48159)
With the stick you can make a spear. Second find a stone. (You can find a stone in the Beach 1 Area. You just continue to walk near the sea 'til you reach a small place with 2 trees. You'll see a coconut down the trees. Pick it up if you want. Then, wait utnil the sea goes back and ... the stone will now be uncovered so you can take it.
After a shortscene you will see, you will learn that you unlocked the weapon option. That means you can now make a spear. Remember, to unlock the weapon option you must have the stick in your backpack.
Now, you press "Make Item". There you will see two new options added in the menu. The "Sharpen" option, and the "Make a Spear" option. After that you have to decide how your weapon must be. So, you can just make a wooden spear with a sharp point. The other weapon you can make is the wooden spear with the stone. You just go to the "Make Item" menu again and you press "Sharpen". After you chose the stone, you press make and you have a sharpened stone. You no go to the "Make a Spear" option. You chose stick + sharpened stone, press make and there you go! Your spear is ready!
*How to kill animals, where to find animals, can we eat the animals? (Check Animal Hunting Arcticle to see).