why do you want to unroot your phone? the only reason i can think of is for repair as rooting voids the warranty if i am not mistaken.
you will need a backup copy of the original stock rom or a downloaded copy of the stock rom and this needs to be installed. as far as unrooting i believe its possible but i'm not sure of the exact process.
you might want to poke your head around xdaforums or do a google search for how to unroot an android phone.