1. You will need to obtain a divorce from your first wife, then re-marry your 2nd wife.
2. You will need to refile with Immigration and write an explanation of what happened o0f why you did not disclose first marriage, and include with your Divorce Decree from #1 Wife
You're not the first and won't be the last this happens to a person, but your attitude at time of interview on this matter and why you did not mentioned first marriage in the petition could make or break a visa being issue for your 2nd wife.