How to get a divorced?

Sir, I am working in a private school and i have a 1 year baby girl. I found in previous year that my husband is having an extra marital affairs with a lady working in his office. At that time my baby is only 1 month old but i am suspicious about his behavior earlier, but didn't find any prove. One day i find a selfie photos of them by grasping each other but he took the mobile forcely and erase the photos. She came to my home also on an occasion of my baby shower with her friends and her mother. I got angry and helpless, i sent many bagging message to her to leave my husband. She replied that she is not interested in my 2nd hand husband. But they continue to talk and chat each other, i found many photos of her in my husband mobile afterward also. My husband put security code in his mobile that i can't get any information. Its completing 1 year but they continue their relation and my husband didn't stop and feel sorry for his steps. As well as he told me got court marriage before i got married. But how far its true i dn't know. Please advice me.