I no longer recommend using this resource and instead recommend using this: https://github.com/RageMpOpenSource/RAGE-Accounts
This gamemode was never a great solution but was intended as a starting point, this does work but if you want to benefit your server you either need to look into improving the gamemode or use the other one I've recommended.
This is a template of a very basic login/registration system you can implement into your gamemode to get started. This template only stores usernames and passwords(encrypted with BCrypt) so anything extra will need to be implemented yourself.
Either download the source from RageMP or by getting it from here. You'll also need to have a MySQL server setup, either using WAMP/XAMPP/or from a server. To keep this short I will not go through setting these up, simply Google 'How to setup WAMP' for example to get it setup.
I'm more than happy for anyone to contribute and fix up anything, and I'm happy for anyone to create Issues if there's a fault they found/any suggestions to make this better.