If you already have your ID number, you can follow the account-setup directions at https://barnard.edu/studentcomputing/setupsecurity/newstudent
You can find helpful info and answers to frequently asked questions on the Student Computing website. You'll also find contact info, in case you have additional questions about your new myBarnard account and/or your new Barnard email account.
Important Note: As with all of your email accounts, be wary of phishing emails that ask you for personal information. Remember that neither Barnard nor Columbia (nor, for that matter, any reputable business or organization) will ever ask you to provide your login, password or Social Security Number in an email. DO NOT REPLY to any mail that does ask for such personal information. If you have questions about any email that you receive which asks for such information, please email Student Computing.