One ram slot is on top of the other and heres a video to help understand
https://www.youtube.com/watch?v=rW_uqf6KhkY
ram can be bought on ebay or amazon or here on ifixit just make sure the ram is matching (both dims same speed )And that its DDR3 SDRAM with a proper pin count of 204 and speed should be between 1066 to 1600 anything beyond that requires overclocking in the bios
Hope this helps