You would use an <a> tag. Your syntax would look like this,
<a href=”linktoyourfacebookpage.htm”><img src=”path/to/your/button/image.jpg” /></a>
This is how you would make the link if you were doing it with an image. If you were doing it with text you would write it like this,
<a href=”linktoyourfacebookpage.htm”>Your Link Text</a>
I’m not sure how you have your site built. If you are using a CMS, you can add links much easier. If your site is custom made, you will have to add the above code to the source code on the page you want the links on.
Hope that helps,