Trying to access my old files but had to create a new Apple Account.

I recently tried accessing my 2011 iMac to find some old photos and I've all but forgotten anything about Macs apparently.. and I didn't know much to begin with. The computer would not boot up. Progress bar we get about a quarter of the way and it would shut off. Eventually I tried updating the operating system by putting it on an external hard drive and rebooting from that. Instead of being asked to log into my existing Apple ID it seemed I was being forced to create a new one. I tried backing up to see if I had missed something but apparently I had already gone too far because when I restarted the only option I had was the new one ( I had read online that creating a new Apple ID would not erase my old data, but about halfway through I started to get too concerned about it and try to abort).

So the computer now works but I only have the new Apple ID and I'm trying to go to system preferences > users and groups then I'm walking with new admin password and clicking the + to add user as I read online but I never get to a place where I can enter in my old Apple ID and password. It's only creating a brand new user with full name, account name, password, verify, password hint then Create User.

I'll also add that if I go to finder and select Deviced > Macintosh HD > Users then I can see a folder where at least some of my old data was stored , but the files are all locked because this is I don't have permission to access them. Hence the reason why I'm trying to add another, existing Apple ID so that I can access them. At this point it just feels like a strange mix of a bunch of random data I've read in other forms and elsewhere online and like I said, I'm pretty Apple ignorant. Is anyone can help me add my old Apple ID to this old, 2011 iMac with an updated OS, which is showing as Mac OS X 10.7.5, I would be so grateful. Or if there is another work around at this point to access those locked files that would be great as well. I'd prefer to be able to switch users between my old ID and my new id. The problem is that it seems to be difficult to find information that applies to this old machine. A lot of the help that I find online is for newer Macs.

Thanks much All!!

[Re-Titled by Moderator]