Computer Science Networks Help?

It depends on the network protocol. You can have token passing for a ring topology (though nobody uses it), CSMA/CD (e.g. ethernet), CSMA/CA (wifi), switching (most networks where you want decent performance) etc., for local networks. For WANs you use something called routing so each router knows which of its neighbour routers it should forward a packet to so that it gets to its destination fastest. Link state routing involves creating a graph of the network to determine where to forward packets. Distance vector uses a distributed algorithm to determine where to forward packets to and is used between ASes (ISPs etc) whereas link state routing is used in ASes and in more complex LANs.

As an example with CSMA/CD if you want to send data you listen to the line, if nobody is sending anything you start sending your frames of data, if you notice somebody else is sending while you are sending you stop sending, ensure the other sender stops by sending a jamming signal, then restart at a random time in the future. If you interfere again you restart at a random time later again, but multiply that time exponentially.

One computer may be running a program that listens for messages and responds to them, which we call servers. A client computer sends a message to the server and listens for the response. There are protocols such as DNS for determining the IP address of the server if you have its domain name, in the example case of if the server is running a web server program and the client is running a web browser. This IP address is used for routing and can be converted into a local MAC address if necessary using more protocols like ARP.

There's a lot of content in networking, lots of exceptions, lots of alternatives, lots of fudgery and lots of protocols.