If you have a Wii that is already softmodded you can simply use something like wbfs_file.exe to convert you .iso file into a .wbfs file. And then you put that into a folder named wbfs on the USB thumb drive. Or you can use a manager like Wii Game Manager to do that on the fly.
Than you put the thumb drive in the 1st USB slot (right one if the Wii stands upright and you look straight onto the back, lower one if the Wiilies flat).
Now you should be able to use any USB loader like Configurable USB Loader (aka Cfg Loader) to play your game(s).
If you need to softmod your Wii first look into the stickies in this sub-forum (Wii-Hacking). I think the ModMii for Windows Guide is recommended at the moment.