how to formet my computer only black picture

need more information.

What is the system doing or not doing? What OS are you using and do you have installation media (like a Windows Disc)?

Typically you put in your windows disc, boot from it, and then delete all partitions before reinstalling the OS. Note that formatting your computer will delete all of your software and personal files from the drive. This is not suggested unless you have no other options and you may want to get any important files off of the drive before going forward with this.