What you need is a router. There are dozens or hundreds of routers available from various companies, and with varying features. You can buy one at your nearest computer store, or even Wal-Mart.
If you already own an ethernet hub or switch, your cheapest option would be to buy a stand-alone router, which you will plug into your cable modem, and into your switch. You will then plug each laptop into the switch.
If you don't already own a switch/hub, you'll want to buy a router that has a built-in switch. These are quite common, and probably don't cost much (if any) more than a router without a switch. You'll plug the cable modem into the router, then you'll plug each laptop into the router as well.
The most flexible option will be to buy a router with a built-in wireless access point. This will allow your laptops to connect using their built-in wireless network adaptors, and you won't need to use an ethernet cable at all (except between the router and modem).
For more detailed information about what types of routers exist, I suggest doing a simple google search for "SOHO Router", and reading up on them before going to the store.
You can also likely buy a router directly from your ISP. This will likely be a router/modem combination, which would replace your existing modem. You'll likely pay more this way, but you'll be guaranteed that it will work with exactly the type of Internet service they provide.