cant get online

Suggest you call your Internet Service Provider's Tech Support. Usually they can walk you through the steps necessary to get online.

 Cheers, Tom 😉

You have a reply to your original post on this topic; so
that still exists in the original location, as well; for now.

Also this issue you note, in an error or dialog box:
*+"settings have been changed by another program"+*
has bothered other users at different points in time
and has its own possible cures, and they may apply.

A google search of those words, will give you several
online user forum questions and replies to that topic.

Since the suggested steps vary, and sometimes they
did not work at first attempt, I will not suggest any here.
This topic has also been addressed in Apple Discussions.

Good luck & happy computing! 🙂

kmoo1302 wrote:
it shows connection to my router, but says it cannot connect to the server.

Do you connect to your wireless network or not ?
Open the Network System Preference and tell us what your IP address is. Is it like 169.x.x.x ? or more like 192.168.x.x ?
"showing connection to your router", if you're attempting the connection by wireless means, may just be the iMac showing the network signal strength, and you haven't connected to anything. The format of the IP address will confirm whether you're connected or not.

While you're in the Network System Pref, is there anything in the DNS Servers box (about 2 lines below the IP address) ? Write it down and remove it if it is. It could be DNS servers for the previous owners ISP, you may be attempting to connect to them to translate web addresses and it may be ignoring you. If you leave it blank, it should pass all DNS requests on to the router. It may or may not have them set, if not, all DNS requests are passed onto the ISP, and so on.