How do you complete Mario Kart Wii

It depends by what you mean by "complete." Do you mean get all characters? all karts? all gold? all two-star rankings? Here is how to get all the karts and characters::: Unlockable Characters:
King Boo - Win 50cc Star Cup
Diddy Kong - Win all 50cc Cups
Baby Daisy - Attain at least 1 Star Rank for all 50cc Wii Grand Prix Cups
Mii (Outfit A) - Win 100cc Special Cup
Mii (Outfit B) - Unlock 32 Expert Staff Ghost Data in Time Trials
Mii (Outfit C) - Get 15,000 points in VS mode - *unconfirmed & probably fake
Dry Bones - Win 100cc Leaf Cup
Bowser Jr. - Attain at least 1 Star Rank for all 100cc Retro Grand Prix Cups
Daisy - Win 150cc Special Cup
Dry Bowser - Attain at least 1 Star Rank for all 150cc Wii Grand Prix Cups
Funky Kong - Unlock 4 Expert Staff Ghost Data in Time Trials
Baby Luigi - Unlock 8 Expert Staff Ghost Data in Time Trials
Birdo - Play Time Trials on 16 different courses/Get 250 wins online
Toadette - Play Time Trials on all 32 different courses
Rosalina - With a Super Mario Galaxy save, win a series of Grand Prix cups/Without Galaxy save, Attain at least 1 Star Rank for all Mirror Grand Prix Cup Unlockable Karts:
Cheep Charger (small kart) - Attain at least 1 Star Rank for all 50cc Retro Grand Prix Cups
Blue Falcon (small kart) - Win Mirror Lightning Cup
Rally Romper (small kart) - Unlock 1 Expert Staff Ghost Data in Time Trials Turbo Blooper (medium kart) - Win 50cc Leaf Cup
Royal Racer (medium kart) - Win 150cc Leaf Cup
B-Dasher (medium kart) - Unlock 24 Expert Staff Ghost data Piranha Prowler (large kart) - Win 50cc Special Cup
Dragonetti (large kart) - Win 150cc Lightning Cup
Aero Glider (large kart) - Win all 150cc Retro Grand Prix Cups Unlockable Bikes:
Quacker (small bike) - Win 150cc Star Cup
Bubble Bike (small bike) - Win Mirror Leaf Cup
Magikruiser (small bike) - Play Time Trials on 8 different courses Rapide (medium bike) - Win 100cc Lightning Cup
Nitrocycle (medium bike) - Attain at least 1 Star Rank for all 100cc Wii Grand Prix Cups
Dolphin Dasher (medium bike) - Win Mirror Star Cup

Twinkle Star (large bike) - Win 100cc Star Cup
Phantom (large bike) - Win Mirror Special Cup
Torpedo (large bike) - Unlock 12 Expert Staff Ghost Data in Time Trials Misc Unlockables:
Picture option to send to Wii Message Board - Complete all 8 Cups in a class
Karts & Bikes (50cc) - Earn trophies in all 50cc Grand Prix Cups
Karts & Bikes (100cc) - Earn trophies in all 100cc Grand Prix Cups
Mirror Class - Win all 150cc Cups
After you get all gold in 50 cc, 100 cc, 150 cc, You get Mirror Cup. Its really complicate if you ask me.