When I plug in my headphones into my imac only the right side plays music. I tried with other headphones and still has the same problem. I tried the headphones with other devices and they work properly. Can anyone help me with my problem please?

Macs have crazy headpne jacks in different models.

So we know more about it...

At the Apple Icon at top left>About this Mac, then click on More Info, then click on Hardware> and report this upto but not including the Serial#...

Hardware Overview:

Model Name: iMac

Model Identifier: iMac7,1

Processor Name: Intel Core 2 Duo

Processor Speed: 2.4 GHz

Number Of Processors: 1

Total Number Of Cores: 2

L2 Cache: 4 MB

Memory: 6 GB

Bus Speed: 800 MHz

Boot ROM Version: IM71.007A.B03

SMC Version (system): 1.21f4

AS BDAqua noted, not all Mac models are the same in all things, not even all iMacs. So, if you provide the information requested, he may be able to provide specific information relative to your specific model.