Do u have the original install disc? Reload OS from scratch.
If u don't, and I assume you have no original nothing on-hand since u asked, u will have to download ML from Apple, then create a bootable USB HD or thumb drive with it (8G minimum), boot with that (USB) then run disk util and erase everything (with partition) on the Internal drive, THEN re-install OS from scratch from the USB HD/thumbdrive.
Here's an excellent article on how to do that.
you should have a bootable volume already. hold down Option right after the start up dong, see if an orange Recovery volume appears.
when i reboot and hold option R do i just choose reinstall mountain lion? will that delete all my stuff first?
No that won't delete everything. While in that recovery screen start Disk Utility and erase Macintosh HD. Then quit Disk Util and proceed with the install. That will give you a system with nothing but the OS.
It will take some time as the OS will be downloaded from Apple's servers.
ok thanks alot for the reply and by clicking install you mean clicking " resinstall mountain lion" right?