You will need to replace your existing broadband modem with a router. This will enable you to share the broadband connection between the two PCs.
You don't mention if your broadband is ASDL or cable but the same applies to each.
I am sure there are USB routers out there but I don't bother with them. Go Ethernet. Most routers are geared for this and many PCs have Ethernet built-in these days. Otherwise it's a simple case of adding an Ethernet card to each PC. They are cheap and it's a simple procedure.
I'm assuming the two machines are located apart from each other so you may want to consider a wireless router for the remote machine to avoid having to run a cable.
Hope that helps with the basics.
[Sorry - we crossed spanners !]