To bring up your "Commands bar", press the "C" key. This can also be changed in your settings to any key you want. When the "Commands" bar pops up, type these codes below. This only works for single-player mode. You also have to enable cheats when creating a new world.

Effect	Code
ascend	Moves you to the next platform above your position.
atlantis [on/off]	Toggles "Atlantis Mode".
biome	Tells you what biome you are currently in.
bring [entity]	Brings a specified entity to you.
cannon [strength]	Shoots a primed TNT block toward where you're pointing.
chest [drop/get/fill/swap]	Access commands of chests.
clear	Clears the console.
clearwater	Toggles the clarity of the water on/off.
clone [amount]	Clones the block being pointed at.
confuse [distance]	Confuses mobs.
cyclepainting	Cycles through the painting you are pointing at.
damage	Toggles player damage on/off.
defuse	Defuse all nearby TNT that is active.
descend	Moves you to the next platform below your position.
destroy	Destroys the current item.
difficulty [0/1/2/3]	Toggles difficulty from 0 (Peaceful), 1 (Easy), 2 (Medium), and 3 (Hard).
dropstore	Transfers everything from your inventory to a chest, which is spawned next to you.
duplicate	Duplicates and drops the current item stack you're holding.
explode	Sets off an explosion in your area.
falldamage	Toggles player fall damage on/off.
firedamage	Toggles player fire damage on/off.
flammable [block] [catch/spread]	Sets the specified block at a specified flammability level.
fly [speed]	Allows the player to fly at a specified speed.
freeze	Freezes mobs so they can't move or attack you.
goto [name]	Go to a waypoint.
grow	Grows all wheat and spruce trees on the map.
heal [health points]	Heals the player at a specified number of health points.
health [min/max/infinite]	Refills health to a specified level.
help [command name]	Gives more information on the specified command.
home	Teleport to the spawn point.
infiniteitems	Toggles infinite items on/off.
instantkill	Instantly kills the animal or monster you hit.
instantmine	Toggles "Instant mine" on/off.
instantplant	Instantly grows what you plant.
itemdamage	Toggles breakage or usage of an item on/off.
jump	moves you from where you are to where you're pointing.
keepitems	Keeps your items after you die.
kill	Kills the current player.
killall [mob type]	Kills a specified type of mob.
killnpc [all/animal/monster]	Kills specified living things nearby.
light	Toggles permanent lighting on/off.
listwaypoints	Lists all waypoints on the map.
longerlegs	Makes your legs longer.
mobdamage	Mobs can't give you damage.
platform	Puts a glass block under your feet.
rename [command name] [newname]	Renames a command name to a new command name of your choice.
ride	Allows you to ride the living creature you are pointing at.
setspawn [x-coordinate] [y-coordinate] [z-coordinate]	Sets the spawn point to a specified coordinate.
setspeed [speed/reset]	Sets the speed in which the player moves to a specified speed/resets it.
skin [username]	Change the player skin to any Minecraft skin.
slippery [block name]	Makes the specified blocks slippery.
spawn [name/ID/random/list] [quantity]	Spawns an individual creature.
spawnportal	Spawns a portal nearby.
superheat	Turns all items into their smelted form. Ex.: Sand turns into glass.
tele [x-coordinate] [y-coordinate] [z-coordinate]	Teleport to a specified coordinate.
useportal	Automatically teleports you to the Nether. Use this command again to go back.
waterdamage	Toggles player water damage on/off.
weather [lightning/thunder/rain]	Changes the weather.
world [load/save/seed/new/exit/list/backup]	Controls your game save from the "Command" bar.