How can i file for the custody of my children

My son is 5.5 years old and daughter 1 month. I have been living separately from my husband at my mother's house with my son since my 6th month of pregnancy with second child who is now 1 month old. My mother helped me during my difficult pregnancy and thereafter with physical support since last 5 months. My husband visits my kids once a week or twice as he is running a small business where he keeps busy. The constant agony for the last 5 months wherein my husband fought with me created huge stress for me during my pregnancy and thereafter, thus affecting my health. My basic and peaceful demand was to raise up our children in a better environment and so shift another place locally but my husband constantly doubts my intentions thinking that i am trying to separate him from his parents, which makes me doubt the sanctity of our relationship any longer. I am worried for the custody of my son who is willing to live with me only and too attached. I dont intend remarriage