Can't format my iMac

[DavisP]

Hello!
I wanted to format my Mac's hard drive but couldn't because the disk utility didn't allow me. The erase and all was gray and uneditable. What I know is that I can't format the same drive I'm using or something, right? I tried to launch the OS X Lion app but it didn't work.

How can I make this happen?

 

[simsaladimbamba]

You can't format the volume you are currently using to run the OS on, from which you want to delete the volume it runs on.
You have to use Disk Utility from inside the Recovery Partition (if that is what you mean with "Lion app", as Lion is an OS and not an "app".
OS X Lion: About Lion Recovery
Lion Recovery Disk Assistant