Zelda Twilight Princess is a must if you are going to get a wii.
Warioware: Smooth Moves is a light-hearted game that is a lot of fun (especially with friends.)
You get Wii Sports with the system if you buy it, that game is fun.
um... Super Paper Mario is a platformer/kinda RPG that is a lot of fun as well.
These are my personal favorites- there are other games that are good too, but Zelda, Warioware and super paper mario are the big ones.
I hope i helped.