Ipod Video

I recently treated myself to an ipod Video 30gb and i have got all my music and photos onto it but the videos i have already on my computer for some reason arent in my itunes library. and when i do get one on into the library it says it cant get the video file onto the ipod. i know its something to do with the file format, (e.g Mpeg, div x) i just thought i could ask for a bit of advice on how i should go about this as i am realy stuck...!