General synonyms:
* Timid
* Bashful
* Modest
* Retiring
* Unassuming
* Reserved
* Introverted
* Self-conscious
More specific synonyms:
* Diffident: lacking confidence in oneself
* Hesitant: unwilling to act or speak for fear of the consequences
* Awkward: lacking grace or skill in social situations
* Inhibited: restrained or prevented from expressing oneself freely
* Coquettish: playfully flirtatious, but often in a shy way
* Wallflower: a person who is quiet and unobtrusive, especially at social gatherings
Informal synonyms:
* Nervous: anxious or uneasy
* Crabby: irritable and bad-tempered
* Uptight: excessively anxious or tense
* Butterflies in your stomach: a feeling of nervousness or excitement
The best word to use will depend on the context and the specific meaning you want to convey.