confidence is the sexiest thing when having sex! be flirty, and try to roll play a little! if you have to, watch some porn and see the kind of things they say. it may be awkward at first, but once you get the hang of being freaky it'll come naturally!
i say all kinds of things lol
"you like that wet pussy baby?"
"oh your dick is sooo good"
"i cant wait to squirt all over your juicy fat dick, and lick it all off"
"mmm baby i cant get enough of that juicy dick"
"mmm baby you fuck me so good"
"fuck me hard baby"
"give it to me"
"make me scream your name baby"
"make it clap"
"mmm baby i just love suckin that sexy dick"
there's a ton more, but i cant really think! it's just the spur of the moment kind of thing! try to take notice in what sexual actions he loves the most and talk about those. for instance, my boyfriend loves how i suck him up.. and he loves when he cums inside me, so i tend to talk and moan while suckin him up. and talk about how i want us to cum at the same time!
good luck!