How we can start a process if doesn't exists before?
Do you mean - how do we login from the shell? su and sudo can do this for you, for example
Or:
Normal shell operations produce new processes all the time. ex: grep 'hi' somefile
In order to run the grep executable, the shell creates a child process. The $( ) and ` ` and () constructs make new processes as well.
Beyond that I don't know what you mean.