After defeating the giant goblin boss at the end of Stage 1, use a bomb to blow up his bouncing seed bowl. A cat will appear and give you a 1 Up.
Go to the pond on the first level, then shoot the frog to reveal the genie.
At the Player Select screen, hold X + Y and press B, B, B, B, A, A, A, A, B, B, B, B, A, B, A, B, A, B, A, B, Start.
Here are where most of the secret people that give you items are:
In the first level, you need to hit the frog. In the second level, you need to defeat the goblin that is throwing bombs from a boat. In the third level, You need to shoot behind a gravestone. In the fifth level you need to shoot the eagle. In the fifth level, you need to shoot at a fireplace.