We have no idea which iMac you have or which OS X it is running...?
For Leopard and earlier, see > Adding a printer to Mac OS X v10.5.8 or earlier
For Snow Leopard, see > Mac 101: Printing (Mac OS X v10.6)
For Lion, see > Mac Basics: Printing OS X Lion
For Mountain Lion, see OS X Mountain Lion: Set up a printer