I can only advise on the technicalities of adding a standard footer...
Firstly, how is the site built, i.e. are you using a web page creation package like Wordpress? If so, I cannot be specific as I've never used a package. You will need to check whatever documentation is available for the package.
However, if the web pages are written manually as HTML files using a text editor, it is quite straight forwards. But I must stress that, if you have 600+ existing pages, it is going to be a major job. I'll explain why...
What you need to do is:
a) Check that your web server supports PHP.
b) Set up a separate development area. If your existing development area is on-line this should work as before. But if you work with off-line copies of the web pages, you will need to set up a local host. The best way to do that is download WAMP from the internet, and follow the instructions.
c) Copy the existing web pages to the development area, and rename them with the file extension ".PHP" instead of ".HTML".
d) Amend ALL links between the pages to match the new names (with 600+ pages that is going to be a major piece of work!!!).
e) Test the new pages to see that they work as before. (At this point you can test the pages as you would HTML ones).
f) Create the include file in the development area, and call it, say, "footer.html".
g) Remove the footer info from each web page and replace it with:
<?php include 'footer.html'; ?>
h) Test the amended pages. At this point they have to be on a local host or on-line.
i) Make sure that you have a copy of the existing live site readily available, so you can reverse the update if it goes wrong!!!
j) If all is ok, copy the amended files to the live area (deleting old .HTML files).
I hope that you now understand that, whilst using include files are a good idea in the long term, implementing them on a site with 600+ pages is not a trivial exercise. You'd probably be better off just doing the changes the old way for now...