It would probably be good for you to ask specific questions on the things that are blocking you.
However, here is the basic idea:
You need a page with a couple of text input elements for UserId and Password, which could be named txtUserId and txtPassword, and a submit button for the user to click. The page can submit to itself, which is sometimes called a postback.
In the code of the page you need to read the form variables for the two text input elements, read from a file and compare the values from those variables against the data in the file.
To read values from form variables you use the request.form("txtUser") syntax.
To read from a file you will want to use the FileScriptingObject and a TextStream object. You can search these on the web to find lots of tutorials and examples.
You will need some way to store the allowed users in the file, but for now I would suggest starting with just one user. If you store the UserId on one line, and the Password on the next, that will probably be easiest for starters, then you can make it more sophisticated after you get that figured out.
Once you get the values from the file, you will compare them with the values that came from the postback, and then allow access if they match.
Of course... what does "allow access" mean? You will need to authenticate the user on each request for each page in your site. One way to do this is by checking for a value you store as a session variable. Lets call this session variable "Allowed". When someone successfully logs in you set this session variable to "True" like this:
Session("Allowed") = "True"
Once this is done, you can check this as the first action in any page on your site, and redirect to your login page if the user hasn't yet been authorized.
If not Session("Allowed") = "True" then
That is the basics of it. Let us know if you have some specific question.