how can I call a js file?

Hi;

I want to run a statistic on my blog, to do that I have
javascript code which I have put it in a file and save it on a folder , now I want to run a script on the header of page to call the script.
where can I put this script to call the js file?

I have looked at index page, header page but could figure it out which page it should be in and how to write it, because pages are in php and no html code like head are in it. and js should be called in between head tag.
thanks