All modern RAM (memory) is keyed so it can only fit into the DIMM slots a certain way.
With modern motherboards, it should not matter which slot you use, though if they are
numbered in the manual or on the board, it is always a good idea to go with slot one first.
Hold the RAM module next to the slot so that the indentation(s) on the green PCB line up with
the bumps in the slot. (see pic below).