How do I leave a relationship when we have a baby together?

I have been with my partner for 10 years. We have a baby together, but I don't want to be with him (or anyone else) anymore. He puts me down, nags at me constantly, shows me up in front of people and does nothing to take care of our baby. I feel squashed and need to get out. I'm not happy. We both work full time and I take care of our baby too. How do I leave? How can I afford to leave? What happens with custody? How do I know if I am doing the right thing? I don't know how to sort things out financially - we have a mortgage and some debt. I don't have any family around. Thank you for your help. x