Is union-acting only for the trained and experienced actors or is it not suitable for actors that are acting in something for the very first time?

Check out Actors Equity, Screen Actors Guild and this for information.

http://www.actorschecklist.com/union.html

Anyone can say that he or she is an actor, but without a succession of documented roles or experience, you are an amateur. You need a resumé listing your actual roles on stage.

I believe you need to have a certain number of hours of paid acting before you can join Actors Equity. Or a number of jobs. Something like that.

Some shows are limited to those with a union card. If you’re not in the union, you can’t get those jobs. Usually the job notices say union members only. But the other jobs are open for anyone, so do the auditions and hope for the best.