If you’re using WordPress I recommend you use the plugin Google Analytics by Monster Insights.
Else, if you’re not using WordPress, you’ll need FTP access to your website, or access to your web hosting control panel.
Depending on what CMS you’re using, you’ll need to find the template file that generates the <head> portion of your webpages, and edit this file by inserting your Google Analytics code just above the closing </head> tag.
If you’re not sure what CMS you’re using, try using https://builtwith.com/. This should detect your CMS provided your site is not using any security plugins which hide what CMS your site is using (a useful trick to avoid hackers who search the web using automated tools, searching for all Joomla sites, or all WordPress sites say).
Once you’ve found your CMS, you can then work out where your webpage template files live by searching Google. Find the file that looks like header (it might be called header.tpl), and see if the </head> tag lives in that file**. If so, add your Google Analytics code just above this tag, and upload that file over FTP (make a backup of the file on your website first by renaming header.tpl to header.bak.tpl on your website, and if something goes wrong with your website delete the header.tpl you uploaded and rename header.bak.tpl. to header.tpl)
** You can download Notepad++ and search the entire femplate folder for </head> automatically, rather than searching individually through separate files manually.
If you still have trouble at this stage, I’m happy to help (it will take about an hour @ $60/hr). Fill out this form, and I’ll investigate a solution for you.