I have engg. vivas starting from tomorrow. What should I do?

I have engg. vivas starting from tomorrow. What should I do? Hi there, I always have had stutter since when I was 4, which have become worse since 4-5 years. Previously, I had low stutter, but now I've mild one. :( I'm doing engineering, and I'm in second year. Tomorrow, I have external viva. Marks obtained from that will be added to our thoery exams. I know, I'll suck at it. Can someone suggest me how to remain calm and fluent, why answering the questions? I did pretty well in theory exams and I don't want to ruin my overall score by remaining silent. Thanks.