Place a chest or barrel where you want the trash can
Put a hopper under the chest or barrel
Place a dropper or dispenser facing into a lava source, cactus, fire, or a void drop area
Connect the hopper to the dropper or dispenser with another hopper or a short item path
Add a button, lever, or redstone clock to activate the dropper or dispenser
Throw unwanted items into the chest or barrel
Activate the redstone to destroy the items
Use a sign or trapdoor on top if you want it to look like a trash can