How can i get my Windows Media Players files into Itunes all at once? Rather than file by file or folder by fo

I bought a 30 gig Ipod and i want to put music into it, but my music is in Window Media Player format. I have 13,000 songs in my computer and i'd like to put at least 7,000 into my Ipod. I've tried to do it on WMP but the ipod won't play'em. It has to be with Ipod. How can i transfer all my music into Itunes all at once, And wait X amount of hours away from the computer, rather than coming back to it every X minutes because you have to up load several files. Can any one help.