First off, I want to assure you that you don't have to do anything if you are not 100% comfortable with it. Dirty talk isn't everyone's bag, and that's okay. That said, it can be fun, sexy, and a great way to communicate your sexual needs to your partner. So, how do you talk dirty? I have always loved this exercise that Chris Gore explains on Sex Nerd Sandra's podcast. It's a great, easy, and playful way to introduce new terms into your sexual vocabulary and see which ones work for you. We all relate to language differently. Personally, "cunt" doesn't do it for me, but "pussy" works. That's something you need to figure out and share with your partner. Then, once you are comfortable, you can work dirty talking into your sexual play. That's when things get fun - and useful!
I tend to rely on of a piece of dirty talk advice I once heard Nina Hartley give: "Don't tell them what they want to hear, tell them what they need to know."
Porn indicates that we should be telling out partners that their cocks are "so big!" That can feel contrived and uncomfortable. When we use the dirty talk to tell them what is actually going on and what we want, it can be really hot and give us what we need. I find this can be particularly effective when combined with a dirty talk technique I call "sexual reminiscing." It's when you use the memory of a past encounter to fuel the present one: "I want you to ____ me in the _____ like you did that time when we ______ all over the _____ because that felt so good." Voila!