What to do with a 2011 Mac out dated

I bought a iMac 2011 for 600$ yes 600$ and when I tried to install fb messenger and other apps wouldn’t let me. I tried messaging the lady I bought it from and never hear from her . I took it to the computer geeks and they told me it’s out dated like bruh really?. So now I’m stuck with a computer from 2011 and can’t do anything. I mean the computer is big and nice well kept but what do I do with an out dated computer?. I should of done my research first before gone buying it from the lady my first Mac computer at that and didn’t have no idea no clue that iMac go out dated smh.

[Image Edited by Moderator to Remove Serial Number]