I don't suggest this for a professional database as I don't think its the proper way to do it but I suggest this. Create your table containing:
Username
Password
Status (if different users have different access)
Then create a form (you can use the wizard for this) and make it lookup the password, for the username that is typed in, I'm fairly sure you can select that in the wizard too. Set the password box to hidden (visible = false) and also lock it so you can't edit it etc, draw another data entry box for the user to type their password into. Then you need an If statement to see if the password entered into the new box = the password in the old hidden box. If so then load a new form (create a button to open new form, and take the code out of it if you don't know how to do it yourself), and if the passwords don't match, obviously you don't log them in, maybe display an error message.
To get the form to load straight away goto Tools/Startup, in the top right box select the name of the form (say loginpage) that you want to load. Now that'll load up every time you start up the database.