How do i connect multiple computers to the internet?

A wireless router would be the best option. You'll need to get a wireless adapter for the computer downstairs, if it doesn't have wireless capabilities, so that it can connect to the router. This will allow you to use the internet on your computers simultaneously: the one upstairs through wire the one downstairs through wireless.

If you have 2 computers you should get a wireless broadband router and connect your modem to your wireless modem you have to buy a wireless modem for around $50 and get a wireless pci card for your computer and connect it that's if you don't have a wireless device or your computer. If you have a laptop all laptops should at least have have 1. My advice is to get a wireless modem and 2 pci wireless network cards.

Install a wireless access point or router. This can generate local IP addresses for all systems on the local network.

run an ethernet cable down to the puter through the wall. you could try wireless router, not sure if that would work though, unless you get the antenna extenders.