just keep racing!!!!!!! i think that there are a total of 5 to unlock. wario daisy dry bones and two more i think. maybe only one more not quite sure the answer to your qustion is just to race race race!!!
in 50 cc you need to get gold on the first 4 cups. Same with all the others.
I dont no about bikes.
You have to get gold oon every 50cc cup to unlock Daisy and Dry bones.
For Walugi you need to get gold on Retro grand prix all 100cc
For R.O.B you need to get gold on every cup 150 cc.
you have to complete certain things to unlock stuff. this website shows you how to unlock everything, including karts. http://www.videogamesblogger.com/2008/04/18/how-to-unlock-all-mario-kart-wii-secret-characters-bikes-karts-and-more-guide.htm
Get gold trophy in grand prixs to unlock karts and characters.
To get Rosalina, you have to have a New Super Mario Galaxy save file. Also, you have to beat time trials to unlock certain characters and bikes.
Mario can have 8 karts and 8 bikes.
Win all the Grand Prixs[50cc, 100cc, 150cc, Mirror], unlock all characters/karts/bikes and win all ghost races.
As you progress through the game, completing cups, you will unlock the different characters and karts.
You have to complete certain cups, and do certain trials but you can only get them on single player!
You have to unlock every staff ghosts, unlock every characters, unlock every karts and bikes, unlock every cups in all of the engine classes, get all the cups at least one star or more, and also unlock mirror and win that engine class.
To unlock karts and characters, you can get them by doing Grand Prixs or Time Trials, or winning lots of wifi races. Or, you can simply play the game alot.
Loads!
get action replay dsi and select the code unlock all characters, karts and classes
There are 30 different vehicles further split into 15 bikes and 15 karts, and even further split into Small, Medium, and Large categories. There are 26 characters, counting Mii A and Mii B as separate.