How to use a desktop as a router?

I have an old desktop. Its got a modem input. Its online via dialup internet. I have a Highspeed ethernet wireless router but it has no telephone line input. The computer has an ethernet port. I know information is a two way highway. So my question is is there a way for me to use the ethernet port to broadcast the internet through my wireless router? its already plugged in. my laptop is connected to it already but no internet. HOW do I broadcast from the desktop?