Your iMac display is a high quality, IPS display, and can be calibrated quite easily. Any of the top colorimeters will do: Sypder, XRite, etc.

The only downside of the iMac is that it does not allow the display to be dimmed to the same degree as some external monitors. Brightness is a critical part of calibration, so this is a well known limitation. So if you are in a very dark room, it can be a challenge to get completely calibrated. The difference between a factory display and one calibrated even with the brightness setting issue is far bigger a difference than you get from the brightness issue alone. IF you are a hobbyist, in a 'normal' rooms, you should be fine.

Note that you will read lots of posts regarding how you can not calibrate the display, and you will find that many of these are from professionals who require very accurate color, typically for pre-press conditions. These same professionals purchase displays that cost more than your entire iMac, and offer extremely accurate color representation, as well as calibration control.

I have calibrated my iMac (2006 24") with a Spyder 2, and it is far more accurate for photos than it was from the factory. As a result, prints look the way I expect them too, both in terms of color as well as exposure. I recommend that you also use printer profiles if print color is important to you.