Website down due less memory buffer available

Website down due less memory buffer available

My website down due less memory buffer available (linux server php and mysql)
I loggedin through ssh and checked for free space by following command
free -m
and i got below result
It shows only 465 mb free memory out of 12GB. Now I tried to find what
services are consuming most of the memory I executed below command
top
and i got below result this shows total memory consumption to less than 20%.
Does any one know how to find where my memory is consumed or which
services are consuming most of the memory.
Whether I have done it in right way. How can i reduce the memory
consumption so that my website doesn't go down. Is there need to increase
RAM.