If you're unsure, you don't feel confident. It's natural to feel unsure of yourself the very first time you read your poems in front of an audience.
When you feel unsure, you find yourself full of doubt. If you remember how unsure you felt on your first day of school, you may want to befriend that new kid who looks unsure about where to sit in class. You can be unsure of yourself, or unsure how to comfort your friend whose cat just died. When this word was originally used in the 15th century, it primarily meant "not safe from attack."