I was dating this gly he wanted to marrie me to get his green card I find out he been posting hisself on social networks to find woman that will merry him he also been having sex with a underage girl then marry her when she turn 18 for his green card, how can I report this

If you have evidence that he had sex with underage girl, report this incident to local police, and let them take care of the rest. Better than calling ICE.