Okay, I don't mean to rag on you, just want to make you think about this a little differently so go with me here: Say you are posted in the ita thread; and then what? What's the absolute worst thing that can happen in that case? Do you think that anon saying your dress is ugly/your coord is sloppy, means anything in the grand scheme of things? Does it mean you won't ever look good in lolita? Does it mean everyone hates you and you won't ever make friends in your local comm?
And another question: Why do you want to fit in so bad?
Obviously if you're into an obscure subculture like lolita, you want to connect with others who also appreciate it and maybe make friends; let me give you a hint: Doing this on 4chan is probably a bad idea. Sure, we have the friend finder thread, but otherwise anons feel free to shit on others even if they have no valid reason to do so, just because they're anonymous. You won't even know if the anons (hypothetically) shitting on you are actual lolitas, or if they aren't itas themselves.
I understand your worries but at the same time I'll advise you to, again, take it in stride. The less you worry about making mistakes, the more fun you're going to have. And this is what it's about (or at least should be about) - Having fun. For yourself, not for others.
Have fun getting dressed and expressing yourself; if someone has concrit or a suggestion to offer, hear them out, maybe their suggestion will help you dress better, or maybe not, it's up to you whether to act on it or not.
Do you understand what I'm saying? You can't have fun if you worry about strangers' opinions so much, and if you can't have fun wearing silly frilly dresses, then what's the point of doing it at all?