Cannot load computer, Cannot perform a clean install

Hello everyone,

Firstly i am primarily a PC user, so bear with my Mac ineptitude. The computer is a MacBook Pro 15" A1286 model - Early 2012 (i think).

I haven't used my MacBook for a couple of months and when i recently went to power it on (full battery charge) i am stuck with a loading/updating screen.

The bar goes about half way and then the computer simply shuts down - so i cannot even load the OS.
Im not sure whats going on, so now i basically want to perform a clean installation / format drive.

So i follow some instructions online and one article says to hold down 'Command + R' when turning the computer on - which brings up the disk utilty fine.

I then proceed to try and erase the drive, format the drive, partition the drive, reinstall os x - nothing works

I am hit with an error message - "Couldn't unmount disk".

After reading several guides, it seems i might be able to boot off another device in order to be able to perform a clean install by loading a file onto a DVD/Flash Stick - but i cannot download anything off my AppStore account as my Mac is not working.

If anyone could point me towards a workable solution, it would be greatly appreciated.

Thankyou