If something belongs 'to' someone it is their possession. They own it. ie: This book belongs to me ( This is my book) This telly belongs to Robert ( It is Roberts telly) If something belongs 'with' something, it means that it should be with the other thing. ie: This book does not belong here, it belongs with the books in the history section. This screwdriver belongs with the set I bought yesterday. You belong with me in Chinese you might say 你应该和我在一起。She is saying the person should be with her， together, as a pair. If she had said ' you belong to me' it means the other person is her possession.

Simply put: You are the best girl/boy for me.

There might be many explanations about this, I give you two: 1. http://www.english-test.net/forum/ftopic12822.html 2. Don't ask for right grammar to singers; you'll find lots of mistakes. I hope it helps.

It appears you have searched for the phrase in the dictionary, which is designed to give the definition of words. The dictionary does not have every song title and phrase in it. Try searching for each word individually if you do not know what they mean. If you need to search for the phrase, your best bet is probably Google or a web translator.