cheated and idk what to do

so tonight... I let my coworker finger me in his bedroom. it wasn't planned or anything. we were hanging out and it happened.

I know I need to tell my bf but I don't know how to even start talking about it.

if I don't tell my bf, my coworker or his brother will. I'd rather it be me than him for obvious reasons.

please, I know I am the lowest of the low. but just give me some ways to start that conversation.

edit::/ I'm 21, bf is 22, we've been together for almost 4 years.