That depends on which iMac you have. The pre-2006 models this forum covers do not have built-in SD card readers but can use external USB card readers.

Built-in SD card slots started with the Late 2009 iMacs.

This Apple article should help if you have a newer iMac:

http://support.apple.com/kb/HT3553HT3553

We can can tell a lot about your Mac if you do "About this Mac" from your Apple menu. The resulting screen looks like this:

If you tell us what your computer shows for the lines I've indicated with red arrows, we'll have a good start.