If you have an internet router that has more than one Ethernet socket, run a cable from the laptop to the router, or if that's at the other end of the building and you don't want to run another cable all the way, buy an Ethernet switch, connect that in line near the other computer and link the laptop to another switch port.
A switch expands the number of connections available from a router.
If your have a modem-only internet connection that just has a single ethernet port, you need to buy a "cable" style router and connect that to the modem, following the instructions that come with it.
It's the router than gives the multi-device capability; a modem on it's own can only have one thing connected to it.
Most routers also include Wi-Fi.