Hi, welcome to the forums
All you need is a Wireless router, i'd go with a linksys G model. Basically you will connect your modem upto the router.
Then you will connect the router upto your pc using a network cable. The Wii has built in wireless support so you'll be ready to go from there.
Have a look here for more info: Amazon.com: Linksys WRT54G Wireless-G Router: Electronics