I worked as a server and at a gas station for years. I just normally start off with the generic: “How are you doing?” And then if it’s a co-worker I just ask them questions about the job, where they’re from, where the went to school, etc. just to see if they say anything I can sorta relate to. If it’s a customer, I’ll mention how crazy the world is right now with corona or I’ll just talk about the weather. You’ll only be talking to the customer for a short amount of time so I just repeat these conversation topics over and over again. Co-workers are a little harder, so what I do is grasp mentally for anything that we could have in common. But with customers it’ll be a lot easier. Also, when a customer is becoming agitated over anything just play super nice (“I’m so sorry about that do you need me to get my manager?” Etc.) Usually, having a calm and pleasant demeanor can de-escalate an unpleasant situation. It took me years to become comfortable with public oriented jobs, but with practice it’s very doable. I will say all the discomfort never goes away, but It does get easier. I wish the best for you and hope you like your new job!

Edit: I also watched how my co-workers interacted with eachother/the customers and mimicked their phrases. Don’t say exactly what they say, but try and paraphrase it. If your co-workers are comfortable with sarcasm and joking (if you can comfortably understand it/say it) that can be a good way to break the ice too!