I’m 18 and work part time as a cashier for a retail store right now! Before I was hired, I told my manager I stuttered and had a block and she and my co-workers have been understanding. Like many here, I have my good and bad days, but getting a job will definitely help build your confidence with speaking to strangers/peers like it did for me! Most customers are too absorbed in what they’re doing and buying, so I doubt they’ll take the time to comment on your speech (this has been the case for me at least). Good luck, you got this!!