Disk locked, can't install OS

[chores]

I have a 2007 iMac and wanted to do a factory reset.
From Reinstall OS X, the only disk listed to install OS X is Recovery HD, which is locked.
When trying to Erase my hard drive in Disk Utility, the erasing process fails.
My iMac is too old to boot from Internet Recovery Mode.
Any ideas on what to do from here?

 

[chabig]

Are you booted from internet recovery? You can’t erase the internal drive if you’re booted from it.

 

[chores]

I tried but I can't boot from internet recovery, I think my machine is too old.
I also have an external SSD I tried to install the El Capitan on, but I got the machine secondhand so the apple ID isn't associated with the machine. I get the message This item is temporarily unavailable. Please try again later.
I'm also only able to boot from OS X Utilities.

 

[Fishrrman]

A 2007 iMac can't use internet recovery (too old).
You need either:
- the appropriate install DVD
- a USB flash drive installer
- an external drive with a BOOTABLE COPY of the OS on it (you can then "clone this over" to the internal drive using CarbonCopyCloner or SuperDuper).

Which VERSION of the OS do you want to install?

 

[chores]

A 2007 iMac can't use internet recovery (too old).
You need either:
- the appropriate install DVD
- a USB flash drive installer
- an external drive with a BOOTABLE COPY of the OS on it (you can then "clone this over" to the internal drive using CarbonCopyCloner or SuperDuper).

Which VERSION of the OS do you want to install?

Yes, my machine is too old for Internet Recovery.
I want to install El Capitan. I tried making a USB installer using my roommate's iMac, but it was too new to install El Capitan (which is a bummer, because the download took 3 hours). So the only solutions I have are to create a USB installer using an iMac compatible with El Capitan, clone a bootable copy of the OS using an iMac compatible with El Capitan, or use the install DVD?
How would the DVD work, just pop it in and it'll take me through the steps?

 

[chores]

A 2007 iMac can't use internet recovery (too old).
You need either:
- the appropriate install DVD
- a USB flash drive installer
- an external drive with a BOOTABLE COPY of the OS on it (you can then "clone this over" to the internal drive using CarbonCopyCloner or SuperDuper).

Which VERSION of the OS do you want to install?

So my roommate magically has a 2007 iMac, and I was able to successfully create a USB installer.
When I go to Reinstall OS X, this is what happened:
"To download and restore OS X, your computer's eligibility will be verified with Apple."
*Continue*
*Agreed to the terms of the software license agreement*
*Selected Install OS X El Capitan as the disk where I want to install OS X*
Was prompted to sign in to download from the App Store.
*Entered my Apple ID and password*
"This item is temporarily unavailable.
Try again later."
The computer won't recognize my ID because I've never signed in with it before, I got this machine secondhand.

So I guess I'm calling tech support when they open tomorrow?

Edit: figured out how to use the USB installer correctly, currently trying my luck!

Edit 2: the USB installer worked, thanks for your help!!