Online Multiplayer

My semester is drawing to a close, and I think that while I now have a greater understanding of the enormity of the task I have set myself, I don't quite see the path I should take.  One thing is clear, programming effectively and properly will take time to learn.  I am still committed to making this an online multiplayer game, and to that end I studied what it would take to create a basic system where a player logs in.  I found that there are many many different ways that this could be accomplished and I quickly became overwhelmed.  I probably should have found a simple way to do it so that I could pass my C++ class, but what I really wanted to do was to learn to create a flexible and robust database system that would store much more information about players than just their email and password.  Because I don't exactly know what information about players will need to be stored as of yet I am not able to create a data structure for the database to hold.  I am hoping to get more clarification on data structures next semester in one of my classes.  In the mean time I can get back to  working on art work and fleshing out the use case scenarios for everything in the game.