legal separation

I am thinking about filing for legal separation. My husband has been cheating got his girlfriends named tattooed on him . Spends money on her and always is going out with her instead of spending time with his daughter. He told me he doesn't want to spend time with me. And he wants to take the baby with him when he goes to see his girlfriend I am completely against it. He is in the military I'm not for sure where I go from here or how I get started on legal separation