16 and engaged

Me and my fiancée now were dating for 4 years and he proposed on my birthday . The ring is cheap but I honestly don't care it's still beautiful and he's gunna get me a diamond ring once he pays some bills . I'm 16 and he's almost 18 . We plan to be engaged for awhile. I haven't told my mom yet I just don't know what to say to her about it. She really likes him though and she always talks about us getting married and stuff . How do I tell her we are engaged?