As green granite says. Get them both connected to the Internet via your router. Then do the "Start - Run - CMD - ipconfig /all" thing and note the IP addresses of the two machines. They'll be something like 192.168.0.2 and 192.168.0.3. They may start 192.168.something else, or 10.0.something. Whatever they are, you should see two sets of numbers in the same sequence. If you have that, things are looking good.
On the 192.168.0.2 one, type into that same CMD box ping 192.168.0.3 (or vice-versa, etc). You should get a report that talks about "Reply from 192.169.0.3 ...."
If that works, the two machines are seeing each other.
You have to enable file sharing on the folders etc you want to share,and to define how that sharing will work.
You then have to give your network a name, and tell the machines what it is and to connect to it.
Once you've done that, go into Windows Explorer, My Network Places, and look for your network's name. You should see both machines in there.