Adsense code

Sorry, I have asked this question before, but can someone explain to me where and how to install the google adsense code please? I am not a coder, please explain in plain words. I do not know where “header.pho” is. There is no Adsense plugin and the video doesn’t show how to add the code.

Please help. Google rejected my Adsense approval because my site had ‘no content’ because the place where I was told to put it last time was not correct.

Thank you!
One Dad