I have two computers connected to a router, my problem is how do I connect the two computers to play a game.

basically you need to network your pcs so they can talk to each other, one this is dont the rest is very easy.
Im not going to go into details about this- it depends on what operating systems ure using.The one thing you do need to do is to change the tcp/ip settings for both your pcs so that they have an individual ip address (usually 192.168.1.x, where x is 1,2,3, etc- each pc has to be different)
once your pcs can "talk" to each other (share some folders and see if you can copy them from computer to computer), make sure u have 2 copies of diablo (burn a copy or use a hard drive crack) and launch a tcp/ip game in diablo ,multiplayer, it should all work fine.