Minecraft wiki entry for Survival_in_an_infinite_desert, associated text: Go outside and dig up 16 blocks of sand.
Dig out a piece of orange terracotta on the floor. Do not jump in!
Place blocks of sand into the hole until it is filled up.
Dig straight down until you are two blocks away from the floor.
If there are mobs, kill them.
Punch out the pressure plate in the middle of the floor.
Take anything you need out of the chests. Bones are good for bone meal, and any iron, gold, diamonds, or emeralds are also definite needs. Also, remember to dig out the 3x3 layer of TNT underneath the sandstone floor.
Using the pillar-jump technique, pillar-jump your way out.