There are two things you can do:

First, when booking your Visa appointment, did you list your disability? It would help if you mentioned it the next time you schedule your Visa appointment.

Second, when you appear at the window, you can proactively inform the US visa officer that you have a speech impediment. You could smile and say something like, “Good Morning. I wanted to let you know I stammer, so please be patient with me.”

Hope this helps. If you need help with your mock visa interview, check out our F1 Visa Interview services.