Sexual orientation and dorm mates

I'm gay and i'm really nervous about my colleges dorm assignments. How should I go about informing my dorm mate that i'm gay as I don't want him to find out later once we're established in our dorm and him get angry about it. I want to present the information upfront. I'm not sure how dorm assignments work, but some have said to contact them on social media before move in day and tell them you're gay. I just don't want to have much confrontation and I'm not sure the best way to mitigate it. Advice?