I am so, so sorry that you've experienced this horrible event.
Having typed that, you have two choices before. You have the option of not telling anyone to protect your boyfriend and remain a victim to "Josh," and whomever else your boyfriend knows, OR you can call your local rape hotline and end your role as a victim, tonight.
You didn't do anything to deserve what was done to you, but hiding what happened is not going to help you, one bit. You are only 13 and sorting this out, now, will help you to progress and develop into an individual with courage, strength, and resiliency - your decisions will be based upon what is in your best interests, rather than someone else's.
You have options, but you are the only one who can make those choices. I wish that this never happened to anyone, but it does, and victims of this kind of violence don't deserve what was done to them and, because of the false sense of shame involved, they never tell anyone and the rapist goes on and on and on hurting other people until they either land in prison, get too old (or, sick) to continue harming people, or they die.
You have the choice to end this person's violence, now. No, you cannot make what he did to you disappear, but you can call the hotline, press charges, get involved in some trauma counseling, and love yourself as you deserve to be loved.
My sincere blessings of comfort and courage go out to you. You are priceless and valuable in this enormous Universe - never forget that. And, this act of violence does not have to be what defines who you are.