How do you plug a Ethernet cable into a laptop?

It depends on the cable!

If it's a standard LAN/Ethernet/RJ-45/'Giant Phone Cord', it plugs into (usually) the only port on the computer's back that will fit!

If the cable is a phone cable, and you use dialup, you will need a modem. Most computers don't have modems in them anymore, because dialup was officially retired in 2004, and exists now for legacy reasons only. Dialup is not intended for normal internet usage.

If you have DSL and it's a phone cable, you'll need a DSL modem. This should have been provided by your ISP.

If it's a large round cable with a single pointy end (a coaxial cable), then you will need a cable modem. This should have been provided provided by your ISP.

If it doesn't look like anything I described? You need to contact your ISP.