Hi Kreeper 23,
Mahalo has a page on Mario Kart Wii Cheats. However, if by "cheats" you mean codes that can make you invincible or give you extra banana peels, there aren't any for Mario Kart Wii. There's a ton of unlockables, but no actual cheat codes for the game.

Unlockables
Alternate Title Screen and Ending Screen

Completing every single cup with a first place unlocks two new alternating Title Screens and one new Ending Photo.

Characters

Unlockable How to Unlock
Baby Daisy 1 Star in all 50cc Wii-Cups
Baby Luigi Unlock 8 Expert Staff Ghost Data in Time Trials
Birdo Play Time Trials on 16 different courses or win 250 Wi-Fi matches.
Bowser Jr. 1 Star in all 100cc Retro-Cups
Daisy Win 150cc Special Cup
Diddy Kong Complete retro 50cc Cups
Dry Bones Win 100cc Leaf Cup
Dry Bowser 1 Star in all 150cc Wii-Cups
Funky Kong Unlock 4 Expert Staff Ghost Data in Time Trials
King Boo Win 50cc Star Cup
Mii (Outfit A) Win 100cc Special Cup
Mii (Outfit B) Unlock all 32 Expert Staff Ghost Data in Time Trials
Rosalina Have a Super Mario Galaxy savefile and she will be unlocked after 50 races or get 1 star in all 8 Mirror Mode Cups
Toadette Play Time Trials on all 32 different courses or win 1000 Wi-Fi matches.

Courses

Unlockable How to Unlock
Leaf Cup Win at least 3rd place in Banana Cup and Shell Cup
Lightning Cup Win at least 3rd place in Leaf Cup
Special Cup Win at least 3rd place in Star Cup
Star Cup Win at least 3rd place in Flower Cup and Mushroom Cup

Golden Wii Wheel

Use the wii wheel extensively in all the modes, when you go online a golden wii wheel will be next to your name.

Have a star by your nickname

Playing extraordinarily well in Grand Prix races can produce a Star Ranking upto 3 at the end results instead of the standard "C, B or A."

Collecting these stars in all Grand Prix's adds a Star Icon next to your name in WiFi Races and on the Results Screen for your opponents to see.

(Note: Say, if you get a Three star rating in every Grand Prix except one which you got a One star rating in. You would get a One starred nickname until you manage to upgrade the rank.)

Unlockable How to Unlock
Double starred nickname Get a two star rating in every Grand Prix
One starred nickname Get a one star rating in every Grand Prix
Triple starred nickname Get a three star rating in every Grand Prix.

Karts and Bikes versions of 50cc and 100cc

You can unlock Karts and Bikes versions of 50cc and 100cc(like 150cc, but slower) by completing all the cups in that CC.

To activate these versions, press the + button on the Speed Select screen.
Unlockable How to Unlock
100cc(Kart and Bikes) Place 1st, 2nd, or 3rd in all 100cc cups
50cc(Karts and Bikes) Place 1st, 2nd, or 3rd in all 50cc cups

Mirror mode

Reach 1st place in all cups at 150cc class.

Unlockable Bikes

After meeting the requirements, some unlockables may require you to back out of your save file and reselect it. Note: In different countries, the bikes have different names but the unlocking method is the same.
Unlockable How to Unlock
Bubble Bike (small bike) Win Mirror Leaf Cup
Dolphin Dasher (medium bike) Win Mirror Star Cup
Magikruiser (small bike) Play Time Trials on 8 different courses
Phantom (large bike) Win Mirror Special Cup
Quacker (small bike) Win 150cc Star Cup
Shooting Star (large bike) Win 100cc Star Cup
Sneakster (medium bike) Attain at least 1 Star Rank for all 100cc Wii Grand Prix Cups
Spear (large bike) Unlock 12 Expert Staff Ghost Data in Time Trials
Zip Zip (medium bike) Win 100cc Lightning Cup

Unlockable Karts

After meeting the requirements, some unlockables may require you to back out of your save file and reselect it. Note: In different countries, the karts have different names but the unlocking methods are the same.
Unlockable How to Unlock
Aero Glider (large kart) Attain at least 1 Star in all 150cc Retro Cups
Blue Falcon (small kart) Win Mirror Lightning Cup
Cheep Charger (small kart) Attain at least 1 Star Rank for all 50cc Retro Grand Prix Cups
Honeycoupe (large kart) Win 150cc Lightning Cup
Piranha Prowler (large kart) Win 50cc Special Cup
Royal Racer (medium kart) Win 150cc Leaf Cup
Sprinter (Medium Kart) Unlocked 24 Expert Staff Ghosts
Tiny Titan (small kart) Beat 50 opponents during Wi-Fi play or unlock one expert staff ghost
Turbo Blooper (medium kart) Win 50cc Leaf Cup

Glitches
More than one person can be the same character.

First, make P1 choose the character you both want to be. Then unselect the character and move off of it. Move P2 over, and select the character. Go back one screen, then go into the character select again. You should both be on the same character. Both of you select the character. Do whatever you want from there on.

Note: In the character select screen and options, whenever one of you does an animation, both of you will. Also, if P2 chooses a different kind of vehicle than P1, P1 will look strange. Don't worry, this won't actually happen in the game.

I haven't tested this with P3 or P4 yet, but in theory, it should work.

Secrets
Boost when recovering from a fall

When you fall into something, such as water or falling from rainbow road, when you land, if you hit 2 at the moment you hit the ground, you get a temporary boost.

Change Mii Racer weight class

Go into the Mii Channel and change your Mii drivers weight, which will have an in game impact causing you to change weight class and have different cars.

POW Block Immunity

When you see the POW block on your screen and its about to hit you, just do the function you would normally use for a trick to create a shockwave for youself to become immune to the spin out the POW block creates (Control panel on GCN controller, Shaking Wii wheel, etc.). This can be done on both bikes and carts. Even though you don't loose speed or spin out, you still will loose the items you were carrying.

Quick Start

When the countdown starts wait till the 2 starts to disappear and then press and hold the 2 button. So the 2 must still be visible but disappearing.
This will give you a boost at the start of the race.