Concept
To be able to do fishing, the player should have fishing rod and the bait at the same time. Everytime the player try to fishing, the bait quantity is decreased 1 unit. Then, we want to have 2 kind of fish. One is the common fish, other is the rare fish. And there's possibility the fish run away. We want to create 7 probability, and the probability is like this : Common fish (4/7); Rare fish (1/7); The fish run away (2/7).
Execution
1. Create 4 item.
{a} Fishing rod. [ Scope: none \\ Occasion: Only from the menu \\ Consumable: No ]
{b} Bait. [ Scope: none \\ Occasion: Only from the menu \\ Consumable: No ]
{c} Common fish. [ Scope: one ally \\ Occasion: Always \\ Consumable: Yes ]
{d} Rare fish. [ Scope: one ally \\ Occasion: Always \\ Consumable: Yes ]
For the {c} and {d} we add what kind of effect we wanted after it consumed ( HP recovery, increasing Spirit, MP recovery, or anything ).
2. Choose a spot for the fishing activities. Create an event there. Make like this:

3. Copy the fishing event in several places that is possible for fishing activities.
4. Create an NPC that will give or sell Fishing Rod & Bait.
5. Finish.
Credit
I've learned this from Tesmy fishing tutorial
Closing
Wish it helps for the RPG-maker newbie.
