Computer low on memory

[Muizen]

On my iMac OS High Sierra 10.13.6 I get the message:
your computer is low on memory
Please close a few application.

Memory contains 4 x 4GB; in total 16Gb installed.

Questions:
1) In what way can I check how much memory of the 16 GB installed is still available?
2) How do I determine what applications I should or can close?
3) How do I close selected applications?

Thanks for helping me!

Harry

 

[firelighter487]

to check if the system detects all the ram, go to activity monitor, select the memory tab and look at physical memory in the bottom middle of the app.