How would I store BTC balance for my users and let them auto deposit/withdrawal?

I want to be able to have users deposit BTC to a central wallet and store the balance on a server.

-Users have to be able to generate a deposit address at any time so they are always able to deposit.

-Users have to be able to withdrawal at any time and have their balance automatically taken from the central wallet to a wallet of their choice.

Not really sure how to go about this. I've seen a few APIs for Blockchain and such but not really sure where to start. Anyone ever done anything like this before?

via John Doe