Nervousness is a normal response to a stressful situation which can be mild to strong depending on the seriousness of the situation. For example going on a first date may be quite mild, an interview moderate and a shark attack pretty major. The brain sends a signal to the adrenal gland at the kidneys to release an approprivate amount of adrenaline to help our fight, flight response.
Now how to help this. By using mental imagery to practice events the neurons in the brain learn to deal with the pending situation. This way of visualising outcomes is now commonly used by athletes to aid their performance and has scientific backing.
So there you have it, visualisations really do work.