How to login in a ASP.Net Website via Form Authentication

Hello , i am currently working on a pre-build Asp.Net Project in which Forms Authentication is used , and there is no user information saved in the database ... now my question is that "How can i log in to that website ???" , If i use my windows account username and password to login it shows error "Sorry, you are not allowed to access this page" ... please reply me at your earliest .... thanks