boot camp is not showing the os x partition , it is directly booting to windows8 . i want to erase all partitions and fresh install the mac os x to it , what should i do ?

You could try Internet Recovery: OS X: About OS X Recovery - Apple Support

Once there use disk utility to make the disk one partition.

Reinstall OS X and go from there.

Command+Opt+R (Internet Recovery) should start with a spinning globe. You want to use a wired connection when you do this. A wireless connection will be too slow.

You can always install OSX on an external disk first and check if your machine reboots and works. OS X: Installing OS X on an external volume - Apple Support.