Yeah I agree , always say ur new to content, I used to say I'm new even if it was my 2nd run and not too confident with the fights.
I have never seen anybody getting kicked for saying they are new, and I've done Alot of instances. What will happen is either someone or all the ppl will just leave( happens rarely) or they welcome u and start explaining mechanics as u go.
U will come across some rude ppl but those are present in every game to be honest and are not the majority in my experience.
So just say "hey Im new to this dungeon, can we do a full run please and let me kow if I'm doing something wrong". Most ppl will take u through it. What I hate is when new players don't say a thing and end up causing a wipe or 2. Because then u are wasting all of our time including yours.
Good luck with ur gaming and enjoy it,