I have been know to "go easy" on newbies, so I will give it a try.
I am a professor at a big-10 university, and I run my own small Moodle for my own courses.
I have used Moodle for face-to-face, hybrid and online courses. I have never designed a self-paces course in Moodle, if this is what you are looking for.
Students do not "pick" their courses in my Moodle, they pick their courses in the university registration system, and then I let them enroll in my Moodle courses by emailing them a course "key." So typically students get "enrolled" into Moodle courses. So, one way to "restrict enrollment" is to give only the correct students each courses enrollment key.
Maybe someone else will know how to let students "pick" their Moodle courses.