basic Log in form

Hi everyone.. This sounds basic but still I don't know where and how to start. I want to create a log in form that will automatically pop up every time an excel file is open. the form must validate the username and password located in a sheet. The log in form have 2 buttons. Button one to validate the user and password and button two to exit the excel file. Specification is the user log in three times and wrong password or user the excel will close automatically. This is just the basics I want to know I hope you could help me so I could apply this and expand the knowledge you are about to share to me. Im new in this field so any help in any form will be highly appreciated. Thank you. Im using excel 2007