Im pregnant and not showing and just started a new job today how can I tell them?

I'm 21 years old turning 22 in july and I am 5 1/2 months pregnant. I had an unplanned pregnancy but I don't regret making the decision of being a mom. I recently got fired from my job at radioshack because of a disagreement with my boss who used to be my fellow sales ociate he fired me. I feel like I am a single mother because my boyfriend is doing very little to make me feel less stressed. He work because I was lucky to send his resume to a couple jobs. I am so stressed out because I just startred a new job as a car insurance customer service representative. I havent notified my job because I am not showing to much. I dont even know how to tell them. It is a start up company and I dont know how I would be able to quit and not have anyone to take care of my son. As for maternity leave I'm not sure what benefit await me. I am so scared they might fire me. I want to be there for my son and dont feel right leaving him with childcare. Does anyone have any thought words of wisdom. please