how to tell my mom I want to have sex with my bf?

Lauren
I'm 18 and I just started college. My bf and I have been dating for a year. He's my best friend and we're honestly completely in love. Like we've talked about getting married and starting a family after college. 
We're both at the point after a year of dating that we're ready and want to have sex. I want to tell my mom that we are thinking about it and that we want to, but I don't know how. I'm close with my mom so I want her to know that we want to have sex. Help please!