How would I go about divorcing my husband, who is an illegal immigrant from Mexico?

We've been married since October 30th, 2012 but have not been together since May of 2013. We've been living our separate lives in different states. We've both decided that we need the divorce and have nothing to debate over. I will raise my son, who is not his son, and our daughter, with him being able to see his daughter whenever he is able to. I'm taking full responsibility for my kids and he will help with buying her clothes and whatever she needs. We don't want a messy divorce and will continue to have a good relationship with one another because we've been better friends, rather than a married couple.