The Main island: (you spawn here) This island has a chest with all necessary items (lava, water, various seeds, etc), a tree, flowers, and grass to get wheat seeds.

Dark oak island: This island contains a dark oak tree, as well as a few flowers, grass, and spare dark oak saplings

Swamp Island: This island contains another oak tree, vines, and an infinite water source. As well as a lily of the valley.

Savannah island: This island contains an acacia tree and a lot of dirt.

Snowy Taiga island: This island contains a spruce tree, enough snow to make a snow golem, sweet berries, and ferns.

Desert island: This island contains sand (for glass and potential potions) and a cactus

Mushroom island: This island contains a brown mushroom tree and red mushrooms on the floor. It also has mycelium.

Birch Island: This island contains a Birch tree, flowers, grass, 3 bees, and a beehive

Jungle Island: This island contains a jungle tree (with a lot of wood) and cocoa beans.

Any questions? Leave them down in the comments/replies!

Also when you try to open the world for the first time Minecraft might say that the world is experimental and might break upon opening but you can just open it and nothing will happen at all!