when you authenticate you can get the StaffID by changing the select statement
"Select StaffID from tbl_Staff where Username=@Username and Password=@Password"
if your datatable having rows means authentication success, you already done that. if you get the value of
dt.Rows[0].ItemArray[0]
value gives you StaffID .
next execute below statement by givn above value as parameter
"select RoleDescription from tbl_StaffRoles where [id]= @id"
then you can read the role assigned to given user, based on that value you can decide which form to open.