How do i connect computers via a router?

Currently, I know how connect computers via direct connection(eg, Com ports, Printer cable) but i want to connect more computers together and able to get files from each other by connecting to a router.

Currently i have two computers, one connected to the router via LAN while the other is connected to the router via a wireless connection.

Can someone tell me how i can connect this two computers together? I have them both using the same workgroup name, and tried many ways to connect them yet failed.