What might happen to me if I marry someone 17 years older?

I don't know much English. I met an American woman online. We have been chatting for a while by translating. He says he can take me to America with him and we can get married. If I stay married for 3 years, I get citizenship. The economy in the country I live in is bad and there is high inflation. I wonder what will happen to me if I go and get married.
What might happen to me if I marry someone 17 years older?
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