If you don't want to have to send it for repair unless absolutely necessary then you could try using a disk cleaner. I bought the official Nintendo one after I was getting disk errors and it fixed it.
Anyway, a few questions..
Does the Wii play other games ok? If it doesn't then it'd be a general repair, if it does then look into the SSBB repair service.
Does the Wii make loud noises when you put a disk in? If it does then your drive needs replacing, if it doesn't then it could just be a dirty or out of alignment lens.
Is your Wii still under warranty? You can check your warranty on the Nintendo website by entering your Wii's serial number.
If it is then you can send it to Nintendo for repair free of charge. All the details are on the Nintendo support website.
If it is not still under warranty then you still may be able to get it fixed because some Wii's had trouble reading SSBB due to it being a dual layered disk. Nintendo have a free repair service just for consoles that can't play SSBB.
All of the links are for North America but Europe probably has the same things.
If it isn't under warranty and doesn't qualify for free repair then it could cost up to $75 to get fixed.