Unless you hace altered any of the defaults, it should be on already. But if you have installed any third party anti Virus programs, many automatically turn it off.
Open Defender. You can do this from the Control Panel. Click Settings and it should show you that Defender is running in "Real" mode.
If not, my first remark applies, and you may have to consider if you wish to keep the third party program, or use Defender instaed.
If I understand your message, Davehc, Windows Defender might not work if a third party AV has been installed? Would it be the case with my brand new computer? Impossible to open Windows Defender. I have a message saying: “This app has been turned off and isn’t monitoring your computer”. Impossible to turn it on”. When I tried to start it (R+service.mcs + windows defender), I still hav this message: “Windows cannot verify the digital signature for this file. A recent hardware or software change might have installed a file that is signed incorrectly or damaged, or that might be malicious software from an unknown source.” Is there another way to turn Windows Defender on?