Both are correct and have different meanings. "I come from China" means you are from China, it is your native country of birth. The verb in the present tense refers to a fact here. "I came from China" is not very accurate. You can rather say: "Something came from China." It means it's source of origin is China.

I came from China sounds like you had a business trip there or maybe you went on vaction there. I come from China means you originated there.

"I come from China."

You can also say, " Hi my name is Sam and I'm from China"

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