To change/set where a domain points to, you need to do 3 things:

1. Add the domain to your web server, which is typically done via a web host's administration panel unless you're running your own web server.
2. Add the domain to the nameservers you'll be using. This is also typically done via a web host's admin panel (usually handled automatically during step 1), but some peple may use separate DNS hosting services.
3. Change the nameservers that the domain points to in the registry of your TLD. This is pretty much always done via the registrar through which you registered the domain. The nameservers referred to are the ones in step 2.

So, typically, you log into your web host's admin panel, add the domain name, and write down the nameservers the web host tells you to use (e.g. ns1.example.com and ns2.example.com). Then you log into the account you used to register the domain and enter the previously noted nameservers. After that, you just need to wait for the DNS changes to propagate.

However, if you're using a dedicated server that isn't managed by a web host or through a web hosting panel like cPanel, then steps 1 and 2 may be a little more complicated. Hopefully, your mom's colleague knows enough about basic server administration to handle stuff on that end for you, because a novice should really be using managed hosting.