FabricMod_SimpleItem_ModInitializer.java

public class MyMod implements ModInitializer {
    @Override
    public void onInitialize() {
        Registry.register(
            // You want to register an item
            Registry.ITEM,
            // Your item ID will be my_mod:my_item
            new Identifier("my_mod", "my_item"),
            // Your item will appear in Miscellaneous tab
            // There're more settings, take a look at those using
            // your IDE autocompletion feature
            new Item(new Item.Settings().group(ItemGroup.MISC))
        );
    }
}