In general, you can accomplish this by using a network device between the modem device for your Internet access and all of the computers that need to access the Internet. This device is referred to as a "router" and comes in many different flavors, but the common element is the routing functionality that allows many computers to look like a single computer to your Internet service.

Some Internet Service Providers (ISPs) supply a modem that actually has router capability built-in, but you will still need to connect multiple computers to the single computer network jack on the modem. In this case, you can use a different network device called a "switch" that acts like a splitter for your network connections. You connect one port to the modem and the other ports to each computer you need to connect, and it directs communication traffic to each connected device.

Of course, the details of how to install and configure a router or switch may vary with your ISP and Internet service type.