I'm not esaccly positive how to get them but i know that it depends nhow much u play. i have it on wii and dsi XL and all u do is play and it will show u if u have sumtin new!
Mario can have 8 karts and 8 bikes.
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
You have to complete certain cups, and do certain trials but you can only get them on single player!
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.
Win all the Grand Prixs[50cc, 100cc, 150cc, Mirror], unlock all characters/karts/bikes and win all ghost races.
That's completely opinion based.
For the karts the Wild Wing. For the bikes the Mach Bike.
Karts go faster, but bikes can wheelie for more speed.
Just beat all the grand prix cups and youll unlock all the karts and for all 36 karts beat ALL the mirror mode cups
If your playing multiplayer there are options in the corner of the screen or if you use a wii remote in grand prix to allow you to use karts and bikes in 50cc.
you can't
Bikes, you drive like normal karts but they are a little different. You can do wheelies if you jerk the Wii wheel up (but you can't steer during a wheelie) to go back Dow you Jerk the Wheel down. Bikes are sometimes harder to control with two wheels but like I said, are pretty much like Karts