if you Ugraded from lets say lion to mountain lion you woulld need recovery disk assistant
but if you never upgraded it just follow instructions bellow.
restart your computer and hold down command +R *that will give you the options. if you want to keep your files intact then select reinstall lion, Mointain lion etc. if you want to completly delete all your files and re-install go to disk utility select your HD go to erase tab and click erase and then go back (go to tab disk utility top right corner select quit) and then click on re-install lion, Mointain lion etc.
recovery disk assistant instructions:
you need a usb drive 1gb or higher
download recovery disk assistance from here (works only with lion mountain lion). Than plug in the USB and open the recovery disk assistance, from there click next and it will ask you to select a disk choose your usb NOTE: this will format your drive. let it do its thing and then close your stuff and restart your computer do not take out your drive. when it is restarting hold down fn to select your drive (may take a while to load) and then follow the inruction that has the * beside it. if I missed anything please tell me.