if you are inexperieced with wireless encryption then stick with wep64 or wep128.
also, depending on the wireless brand and key-method stick with the following characters: abcdef0123456789
type whatever you want, but pick from those characters, you can use the same characters if you want... write it down somewhere.
set it up on your router and KNOW wich letters are capitals and lowercase while you type it (make sure capslock is off)... perhaps test it on another PC (if you have a wireless PC). if successful then try it on your wii.
this is difficult to explain, without much of a description... i'm assuming that if you misconfigure your current setup that you won't get in trouble by a parent or another user...