Google analytics

How and where to install google analytics tracking code ??
Being a complete beginner, you will have to litterally take me by the hands and show me the way as you would do for let´s say a car seller. Ex : do not tell me : go to the header and paste your code, but rather : go to the cpanel, open http://pub/wp-adm/etc, open the file, you will see a line called xxxxxxxxxx, paste it right after and save it. You see what I mean by beginners ?
One more thing, what iss the diff. between http:// sitename.com and http://www.sitename.com ???
I´ll will be gratefull