Instructions
Start "Mario Kart Wii" and select "Single Player" by highlighting it with the remote and pressing "A" to select it.
Select "Grand Prix" from the single player options by highlighting it and pressing "A" to confirm your selection.
Highlight the "50cc" choice and press "A" to select it. Next, choose your character, vehicle and options. The character and vehicle that you choose have no weight on unlocking new tracks.
Race through the Mushroom Cup and get the gold trophy at the end by having the most points, which means usually getting first place in all of the races. Beating the Mushroom Cup unlocks the Shell Cup.
Beat the Shell Cup, again getting the gold trophy. This will unlock the next cup, the Flower Cup. Continue, beating all of the cups and getting the gold trophy. Once you beat all of the cups on 50cc, do the same on 100cc and 150cc. Once you have completed all cups and won gold in each, you will unlock the mirror courses, as well as all of the hidden drivers and vehicles.