Ford Fiesta engine switches off and on when driving or braking

swtich off on its own. Obviously, one would say its not safe to drive it but most days it can go without switching off whether changing down or just stopping. Can you please help me diagnose what the problem could be and how i can go about getting it fixed. Before, i take it to the garage i want an idea of what the problem could be so i dont get ripped off being a woman and all. I've had the car for 8 months and bought it at auction so the owner never told me of the problems. Many thanks in advance.