What you need is an ethernet switch. They come in 10/100 and 10/100/1000 variants and you should get the one that matches your network speed. If you don't know use this guide to find out what is is on your PC.

If you see 1000/1000 or 1.0 Gbps then I personally recommend this switch or this one

If it's only 100/100 then you can save the money and get this one I've also personally used and loved.

To set it up you'd use the Ethernet in your room to connect to the switch, and then an Ethernet for 1) your computer and 2) the Xbox. Then you also have 2 more ports left.