If you choose this route. Click on the apple in the menu bar, About This Mac, then the click on memory tab. This will tell you the type of RAM you need, and what's already installed, whether there are free slots, etc. Mine says 1600Mhz DDR3, and shows I have 2 x 4GB, 2 x 8 GB. It even has a button for instruction on upgrading (takes you here:
https://support.apple.com/en-us/HT201191 ). When I went to order my RAM I noticed there are different voltages available, which that site didn't talk about (atleast not for my model). Digging around online I found that the Mac doesn't care. It can use whatever voltage RAM you want.