Che cosa significa Can anyone help me figure out if it is polite and common for girls to use おまえ to refer to the one they are talking to? And if 君 is normally used by men to refer to women? ありがどございます！ ?

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Girls on not, using おまえ（お前）never be a polite speaking. It's rude or informal. Please be extra careful when you use it. I strongly suggest not to use it until you are confident about the appropriate situation.

Never use 君 to someone you should talk respectfully either. It's not polite, but a casual form. Never use 君 to call someone higher than you. It'll make you sound very impolite.

The most common polite form for "you" is あなた, but it's still not easy to use it naturally because generally, Japanese tend to avoid using the second person pronouns in conversations. Instead, we use their names a lot or simply drop it from the sentence. Since we use different forms of expression for different subjects, it's not so hard to know what the person is talking about without the subject person.

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> ありがどございます！
ありがとうございます。

But it's not appropriate to use it when you ask something.
You can use it only after someone do something for you. You can't say ありがとうございます meaning "thank you in advance." For that purpose, you can say どうぞよろしくお願いします。instead.