First you need to know what type of Imac you have and what type of RAM does your Imac use. in order to do that you need to access the System profile through the Aplle menu.

First: Go to the Apple menu

Open the Apple menu, and then click on About This Mac.

Second: You need to select “More Info…”

it’s within the About This Mac screen that opens.

Third: Select Hardware overview

here the System Profile screen will open, showing youthe Hardware Overview. The Hardware Overview option in the Imac provides you with all the necessary information you will need to change the memory, such as your Mac model, CPU type/speed, and total memory capacity.

Fourth: Current memory information

Choose in the Contents section on the left side of the screen, Memory. This is where the Memory Slot information appears, showing your current memory configuration. This information helps you determine which slots are filled with which type of memory, and how many slots are still available to install new memory.