How to marry

Actually I and my wife are staying together from past 8 years and we we have all document like bamasha card voter I'd and aadhaar in all that document we husband and wife but 8 years before she came from other place and she is already married and she has 2 child,with 2 child she came with me and we are in love so we stayed from at that day,few years back she written her daughter to her first husband,and her husband also got new marriage and he had another child from her new wife,and one child is with me so its necessary that she should take divorce from him,he is already married and he have a child,so know I thought to get married with certificate,how we have to do,we have all document and her parents also think that we are married,because its long 8 years ago,and everybody knows we are married,so how to get marry know,we don't have any family problem,and my child also don't know this matter,because at that time he was 2 month age know he is 8 years old so pls suggest me procedure and cost and how we have to gerpt married or its not necessary