My computer Windows Security Center say my anti-virus is not own. I already install avast and put in the code?

turn it off altogether if you can mate
if you have your firewall and antivirus dont worry about it

yeah wen you install an diff anti virus or whatever the windows one automatically shuts off so your new one can work!

By “own” I assume you meant “on”.
Make sure you don’t have two antiviral softwares running at the same time. Also you may have to restart the computer or go to the Windows Security Center and disable “check for automatic antiviral”

Just keep using Avast as normal. Windows sometimes won’t recognize it right away.