iMac format

[Beja.Naor]

Hello,
I have imac he had bootcamp, formatted the partition via Disk utillity then tried to unite the partition with the Mac partition.
Because the versions of the Mac was so old it was impossible to unite them through the Boot Camp map for restarting the computer did and came to state that the computer can not get
After Rstrt computer has only the windows partition and says missing operating system
I try to format entirely and can not.
I downloaded a new version of os x yosemite and prepared USB flash disk formatting a guide
I'm up your computer, and click on the alt I only see the partition of the windows and also detects the USB flash disk
How can I format the computer and delete the partition?

 

[Weaselboy]

Hold the option ley at boot and select the USB installer to boot from. Once the install screen comes up select Disk Utility and in the left column select the drive brand name at the very top. Then in the erase tab erase the entire disk to Mac OS Extended (Journaled). Then quit Disk Utility and install the OS.

Of course this will erase any data on the disk.