@ejdelmonico I might explained wrong my issue. I know that Forge has nothing to do with DNS, obviously both are totally different things.
I will try to explain again my issue, and then I will explain the solution I found, just in case someone needs it.
My client has an old website example.com and he told me to develop a new website so I've done that. Now, I need to point the domain to the new server of the new website.
In this new server, which I created with Forge in Digital Ocean, I deleted the default site that creates when I create the server. I did this because I wanted the folder of the site to be called as the domain of the website: example.com. So after that I created a new site with that name. The problem is that, I needed to test the website to make sure everything works well before changing the IP of the domain and I didn't know how to test the website. If I used the IP to launch the new server, because there was no default site, it just showed me an nginx error.
So, to be able to test it, I needed to edit the nginx configuration file on the site panel in Forge.
From this:
listen 80;
listen [::]:80;
To this:
listen 80 default_server;
listen [::]:80 default_server;
That solved my issue and made that site as the default site.
Hope I explained well the problem and hope it helps someone in the future.