Hello Sharron!
You have to drop the item (following
@WhiteEagle12's instructions) and then quickly run away, or it'll just pop straight back in. Taking off all Magnet Rings will help. If you have a full inventory and there is already an item on the ground, dropping an item will cause the item on the ground to enter your inventory. If there are multiple items on the ground, the Trash Can is really useful.
If you get this to work, make sure you come back to pick up the items that day. Every night you leave them on the ground they have a chance of despawning.
P.S. You can use the B button to bring up (and close) the inventory. It's much closer than the + button.
Hope this helps!