I start shaking when i'm nervous. is this a sign of mental illness?

Mindfulness: It is not unusual for someone to have some hand shaking with anxiety and stress levels rising. I would recommend books on mindfulness to help in you dealing with your stress.

Probably not: Shaking is a physical response to being angry or anxious. It is almost universal, with sufficient stress. If it is happening to you under situations that seem not to warrant that level of reaction, you might be more nervous than others you know. Only if it's interfering with your ability to function would i suggest you talk if over with someone, maybe your primary care doctor.