*I have installed sql server 2008 management studio with "use sql server authentication".
    *I have created a database with
        database name:stock
    *In Vb.net 
    created a windows application;clicked on Add datasource;changed d datasource to microsoft sql server database file(SqlClient);clicked on option button use sql server authentication n then provided d user name and password;
    browsed d database and clecked on open;This is when i get a error.
    This is the error:
    "Select sql server database file
    you dont have permission to access this file
    contact file owner or the administrator"

    Plz let me knw the cause of d error and how to solve it

you dont have select permission for this database , try to use windows athentication in vb.net or you can call it trusted connection string , in that string there is no need to give id and pass .

Click Here at this site you can get connection string.