Dirty talk...

My boyfriend and I have been together for a couple of years now, and recently (ish,  within the past few months) he's been asking that I talk dirty to him during sex, and I just freeze! I have no idea what to say for him. He wants me to tell him stories about me doing other guys or doing things to other guys, and it's not that I'm too terribly uncomfortable with it, I just haven't a clue ~what~ to say!!! Any help would be greatly appreciated!