wiping a mac hard drive.

self explanatory, i need soem help wiping my sisters mac HD. she fucked it up, and i wanna do a completely clean instalation.

i forgot to mention..............it wont recognize the HD.......and this time i cant get it to read it.it feeds me sum BS that the HD cant be read cuz it cant be found, but at the same time the HD is spinnin around and actin like its workin.