The best solution would be to get a wireless router, plug in with a cable to the modem and then connect thru cable/wifi both computers. If you can't get a cable to the PC in the second room, you'll need a wireless adapter like this one: http://www.belkin.com/us/F7D2102-Belkin/p/P-F7D2102/
The router theoretically allows you to connect 253 different devices to it (a home router), of course in real life you won't connect that much of devices.
There is a big variety of routers with different features, and a lot of brands too
Check this router:
https://www.asus.com/Networking/RTN10E_B1/
You can get any Linksys, Belkin, D-Link, Tp-Link, Asus