This is going to sound weird, but practice at home. Until you can say the words, outloud, comfortably when you are alone, you won't be able to say them in front of others. Think of things that you like to hear, and start by saying those things aloud. Also, get comfortable with the sex words, body parts, and sex slang. After getting comfortable saying things aloud--begin to ask for the sex acts that you would like to do when you are with your partner. Continue until you are conversing comfortably.

Go ahead and feel embarrassed but make it a fun/funny adventure rather than a serious task. Your significant other will enjoy your laughing at yourself just as much as the dirty talk until you get more used to hearing those words come out of your mouth. I'm sure he'll have no trouble helping you practice! (But, don't do anything you're truly uncomfortable with) Have fun ;-)

I am no prude, but I can't talk dirty either! Previous boyfriend asked if we could have phone sex and got all upset when I explained that I just feel like a complete numpty when doing that kinda stuff! One thing you could try is reading from a dirty book or from saucy online stories, that way you are not left trying to think of what to say and becoming verbally frigid, which is what happens to me when I get asked to 'talk dirty'. And don't forget, you dont ever have to do anything that makes you feel uncomfortable!