how to edit or add meta tags in headder of web page

I first published my site about three weeks ago. I've updated it daily since then and have also done as you suggested in adding my key words to my description on each page. I have also asked Google to look at my site but they want me to verify my site by putting a html file OR meta tag into the first header section. I've had a go but it didn't work properly - can you tell me how to do this?

Many thanks,
Yesh