How can I give myself an item or rare resource?

Of course you can enable Creative Mode when you create a world, or even type /gamemode creative in Chat and you'll have every possible block and item.

But you can give yourself a specific item or number of resources, too, using Tynker mods! See how by following the tutorial in your Tynker dashboard at Minecraft > Diamond is Unbreakable to get full diamond gear. Then change up the mod to suit your own inventory needs!