k first of all you have to find a compatible shifter(shimano and sram for the most part are not) the cable you have on there should work(although i am unfamiliar with grip shifts) but if you wanted to repace cable to, if its old and cracked, you may as well do it well you doing the shifter. i figured out how to change my shimano shifters just by taking it apart slowly thinking things through and not taking apart things i shouldnt have. if you really are clueless about it take it to a shop and watch him do it.