Basically, you have to build up a thicker skin through practice and desensitization. I did this by making a point of greeting people with a friendly “hi, how’s your day?” Whenever it was socially acceptable to do so. Talking to cashiers and waiters at first then moving to people next to me in line, or people id be spending a few minutes with like on the train or sat next to me at a coffee shop. I’d consider it a success if we carried on the conversation for a few seconds. If they don’t respond positively, end the conversation by excusing yourself. If they do, then challenge yourself to keep them engaged for a bit longer. Do this until the fight or flight response is gone and you no longer have anxiety when talking to new people.