To back them up on your computer, you need one of only about 4 dvd drives that can actually read a wii disc. Good luck finding one, as they're no longer made.
The more realistic way is to modify your wii via a "softmod". This involves installing unofficial software onto your wii via various hacks and "vulnerabilities" in the Wii operating system. You can then install various pieces of software which will allow you to insert a Wii disc into the Wii, then the software copies the game onto either an SD card or an external hard drive. This software is sometimes questionable in its legality, and its use is also questionable, depending on where you live and your local laws.
Contrary to other answers, ALL of this software is FREE! Those that sell it are scamming people into paying for the software that is freely available. As for obtaining backups, anyone offering to sell (or even give to you for free) a "backup", .iso, etc of any commercially released game is doing so illegally. So be aware that downloading or obtaining backups for games which you do not personally own is definitely illegal. Even creating backups of your own games may be strictly defined as illegal, as the laws surrounding this topic are actually somewhat contradictory.
Another site to look at is http://gbatemp.net and look in the Wii hacking section.