"Error establishing database connection"

[MySeagull]

Backstory (skip if you don't care):I recently got hired to be a website admin. My real job, eventually, is just going to be to manage content, which I'm totally capable.

However, as I began looking at the site and began compatibility-testing it with all of the major browsers, I ended up finding a LOT of minor problems with the site (about 9 typed pages worth). They want me to try to fix everything I can (grammatical errors, broken links, minor coding problems) before sending it on to a developer for the more major stuff. Well great... I only know basic HTML, not php, MySQL, etc.

I wanted to set up a test environment off-line for the website so that I could make changes, test them, then upload them if they worked well. People pointed me to XAMPP.

The Problem:
XAMPP looks like an excellent program but I have zero experience with this stuff. I basically downloaded the entire website off of the FTP and threw it into the htdocs folder. It looks like it's a mix of html and php. When I go to localhost it loads for a while then says "Error establishing a database connection". I did search the forum prior to posting this... The only thing I found are a couple of posts about WordPress. It's possible that they got the template from wordpress then altered it for their purposes. In any case, it isn't working and I need it to.

Any and all help is appreciated... I build PCs and even work in tech support, but when it comes to stuff like this I have next to no experience. Please explain it as you would to a young child! haha!

Thank you in advance!