It would be helpful to know which model iMac you have and the version of Mac OS X that you are running. What make/model of printer are you trying to use, and how is it connecting to your iMac (wireless, AirPrint, Ethernet)?
Without more information about your specific setup, my recommendation is to verify that your printer is powered ON, it has paper loaded, and there are no error conditions that need to be cleared, then open the System Preferences and select the Printers & Scanners settings to see whether it shows that you have any printers set up for printing. If there are no printers listed click the sign at the bottom of the list and choose the command to add a printer or scanner. If your printer is visible to the iMac then you should be able to set up the printer by clicking on the printer name when it appears in the dialog box, then click on the Add button. This article provides some additional information on troubleshooting printing issues.
If you can't print from your Mac or iOS device - Apple Support
If you continue to experience problems please provide the requested information so that other users such as myself can try to provide further assistance.