How do I spawn an item?

(I'm assuming you want it with a procedure). There's only one entity for all dropped items, called item. It uses NBT to determine which item it actually is. However, in the World Procedures, there's a Spawn Gem --- at X Y Z block, which will spawn a dropped item of whatever you selected at those coordinates, which is much easier to use/configure.

1.7-1.10- /summon Item ~ ~ ~ {Item:{id:"mymod:item",Count:1b}}
1.11-1.16 /summon item ~ ~ ~ {Item:{id:"mymod:item",Count:1b}}