Hi!
What kind of "modem" is this? Is it a telephone modem or something else?
If it is a cable modem or DSL connection device, it may already have an Ethernet connector built in...in which case you can connect that to a cheap NAT router (readily available in stores where computer products are sold) and plug that into (usually) up to four computers.
If you cannot do that, Windows and some other operating systems can be configured to share your connection amongst other computers. To use this, you'd need a cheap Ethernet network hub or switch, network interfaces in each computer and suitable cabling for each station that you will be plugging in.
A web search should give you more information about how to set up your computer's software to do this, and how to hook it all up.
William