I have been married for 15 years, my husband and I lived together for about 18 months and then we separated and then after few months he moved away and we lost touch. I have no idea if he is dead or alive for last 12 years. We don't have any children or property. I didn't bother to check for him earlier but now I want to remarry so I want to divorce him but I don't know how?? Can I get married now even though I haven't divorced him on paper?? He has been out of my life for 12 years now.

Yes you have to divorce or prove he is dead.

Your new marriage would not be legal
* not legally married* and be against the law in the US