Question: I want to send the mail only when creating the new contact through workflow. so what criteria i should set in the criteria field.

Note: if I update the contact no need send the mail.

Answer: 
You can simply use the `ISNEW()` function in your criteria. 

Take a look here for this and other formula functions

<https://help.salesforce.com/HTViewHelpDoc?id=customize_functions.htm&language=en_US>
Or just select "Everytime a record is created" (the first radio option) and set the formula value to True