I would like to implement the remarketing tag into my website
I assume you're asking about AdWords?

Forgive me if I need to ask this but who developed your website? It sounds like you need to have your developer add that bit to the website's code. (If you're actually a developer and I'm talking under your knowledge, go ahead and modify your question!)

Otherwise, if have access to ftp or some other form of modifying files on your server, you can add it yourself. (Before you modify any file, make a duplicate copy in case you have to go back.)

The exact place is going to depend on your templates, located in the craft/templates/ and likely in the _layout.html (or some other file like header.html or footer.html).

If you're running Google Analytics (which is a safe bet, since you're using AdWords), the easiest way to do it would be to add a line to your GA tracking code which probably is already installed.  Just look for ga('send', 'pageview'); and add ga('require', 'displayfeatures'); right above that, as in the example given on the page (assuming you're using their newer Universal code).