How to see programs running in my computer?

To see what programs are running in the background and how much memory and CPU they are using: Open “Task Manager”.

What is the use of how much CPU are they using? How it affects the memory?

There is no relationship between CPU and memory usage. A process can occupy all CPUs of a system but use only a minimal amount of memory. Also, a process can allocate all memory available on a system but only use minimal CPU time.