New hard drive and RAM booted to lock screen for firmware now gray scr

I replaced the hard drive and cable plus RAM (now 8 GB instead of 4 GB).

Used OPTION + R to boot to lock screen for firmware password and input the password.

Lock went away, no chime, and the screen is blank gray. The computer spit out the install disc that came with the computer (put in before startup process).

What do I do to complete the job?

I had not kept OS up to date. Purchased a thumb drive with newer OS. After letting it sit for an hour with gray screen, I put in the thumb drive , got Apple icon, then looked like a programming screen. It said that it was installing then went to flashing ? Again.

I did PRAM reset and started again with the firmware password. When at gray screen, put in install disc, can hear the drive kick in and nothing happens for a minute before spitting out the dvd. Then I plugged in the thumb drive again, and got the Apple, then drive icon to install OS 10.11, so clicked it, then pulled out thumb drive after it began loading. Went to error screen and restarted on its own. Put the disc in, pops it out. Don’t understand why it won’t read install disc. In anticipation of the question, I turned off wireless before shutting down screen / replacing parts.

Started over - at the screen with yellow usb drive icon, “OS X 10.11 Install disc - OS C 10.11.3 Install Disk “. scrolling under it. I choose 10.11 and error message says “ unable to find driver for this disk”