you buy it from the shops
Mario Kart is a racing game featuring all the Mario characters except Geno (super Mario rpg) and all of the others from that game (like I said super Mario rpg)
you just have to keep playing the grand prixs and do well on them
This has all the instructions to unlock everything on Mario Kart Wii: http:/www.cheatcc.com/wii/mariokartwiicheatscodes.html
keep playing until you get it