There arent really any set guides in making anything in any language even PHP.
before under taking a project like this although it is small you would want user security, administration to overlook what is happening.
Although it does sound hard and the more complex you wish it to be the harder and more code will be required.
For starters a good user system which it seems is your main proity in this script.
Would need a few things
How are you going to track the user? (Sessions,Cookies or both)
Encrypting Password (Not really a question its a must with all sensitive data)
It may not sound like help but im sure im sure you more than likely have not thought of the basic information.
PM me if you want a little more help.