This is the reason why you can't select 480p on your Wii. You need component cables. Google component cables for Wii, there are several different ones on the market. You need these! Then your problem should be fixed when you get the proper input signal to your tv.
Just go to a place that sells video game accessories and ask them to direct you to some Wii component cables. I got mine at Walmart for like $20 and they work really good. They hook up to the back of your Wii just like the standard av cables, but to your tv they have like 5 prongs instead of 3; this is what you gotta make sure is in all the proper inputs on the tv. Look at the colors carefully.