I assume you have a working network ?

More than likely the router is providing the IP addresses to all the machines. You technically don't need the address of the router since the data will pass directly between the machines. You also don't need to do any configuration on the router to make it work. You only need to change the router when you want to share outside your house..not a good idea.

So open a command prompt on each machine and issue IPCONFIG. It will tell you the IP of each machine. You also get the router address on this, it should be the default gateway.

Most people use Samba to share microsoft files. Many times this is already installed. People have videos on u-tube showing how to install it.