Opinions ?? How to tell work???

So need opinions ladies I just started a PRN job in a career field I just got out of college for about 6 months ago like I said I am per Diem but I am so nervous to tell me boss I’m expecting and I’m not sure when is a good time or how it works since I’m prn I know I won’t get paid or anything but could they fire me or not have me return after??? Cause I have only been there for 6 months . When should I tell her? And what do I say??