MAMP Installation: Using a pre-existing WordPress site?

Bear with me – I’m really new to WordPress and hosting. You may roll your eyes at my questions but no amount of searching on the support forum or web searches have been able to answer them.

So I’ve followed tutorials on the installation of MAMP (on mac) and I started having trouble at the part where you edit the wp-config-sample.php file with your username and password. Pretty much everything up to this point has been simple for me, but I can’t seem to figure the rest out. I enter what I believe is my username and I get a 404 Not Found Error when I enter it in localhost:8888/(username) on MAMP. I have figured out that it is because I do not want to create a new site, but make changes to an existing site.

I am editing a site that was made by someone else — different computer, different everything. I need to add a few things. I can’t for the life of me figure out how exactly to gain access to this. I can log in on the WordPress website and edit it through that, but I need to get my fingers dirtier.

I also need someone to explain to me the definition of a local host, database, and a server.

Again, so sorry if I can’t even explain my problem appropriately and if I sound like an idiot.