^ 1 eithernet cable can't be divided directly into two or more.
just look in your modem. does it give u more then one eithernet ports, if yes, then you already have the option to connect your laptop via the other port as well as your pc.
and if it gives only 1 port, then the easy and cheapest solution is to buy 1 more eithernet card for your pc insert it in your pc. from it you will need a Crossover Cable connection to your laptop.
then all's fine.
this is the cheapest but not reliable. you will need a new eithernet card for each new connection (huhhh..) and your PC needs to be turned on every time when you want to use network connection from it.
one other solution is a multi-port switch connect your PC's 2nd eithernet card to the switch and get more connections from it. but here your need a Straight Cable connection.
yes it gives your more connections but it's also not much reliable you will still need your PC to be turned on.
one more and much reliable solution is the router, that is connected directly to your modem. and also connected to your end user computer(pc and/or laptop).
and for your laptop the more reliable solution is to have a wireless router because you can also purchase a wi-fi devices for your laptop.
what option is reliable for you. it's yours.