Once you have accessed that mode, enter the Nether and place an ice block on your hotbar.
You can enter Creative Mode to get around that limitation (though you won't be able to update the leaderboards or unlock achievements in that world). If you place water in the Nether, the water will typically evaporate. Investigate the area until you find a chest, which you can open to obtain some diamond armor.
As you play through Tutorial mode, acquire enough blocks to build a staircase that allows you to reach the Minecraft sign in the sky.
