Sexual Assault

Just last night i was sexually assaulted by a guy who i thought was a really good friend ( i had no feelings for him at all ) 
I've talked to my friends about it and they all think i should tell my mother.. but i don't know how to. i know it wasn't my fault but i feel like it was and i don't know how to tell my mom (Both the guy and i are 16) 
my mom is a really understanding, open and wise person but i simply don't know how to say what happened.