Telling my mom

Just found out that my Finacé and I are expecting! He is off at basic training and im living with my mother as I am about to graduate high school in a week. Im already 18 and we are getting married in July which already wasnt easy to tell my mom, but I need advice and help on what to say to her when I tell her. I need her support because the rest of my family may not support as much as I'd want them to. I know I'm going to write her a letter because it's easiest for me to express everything to her, but I need help on what to say? Could someone help, please?